docs(skills): address CodeRabbit review — fallback rules, OS claim, headless greeting

Three fixes from PR #2284 review feedback:

## 1. Fallback merge wording (7 SKILL.md files)

Every SKILL.md told the LLM to merge the three customization files
"in priority order (later wins)" when the resolver fails. That reads
as shallow last-write-wins — but the resolver does structural merge
(scalars override, tables deep-merge, code/id-keyed arrays merge by
key, other arrays append). Following the old wording manually would
have silently stripped base `principles`, `persistent_facts`, and
`menu` items whenever a team override was present.

Expanded the fallback sentence to restate the four structural rules
explicitly, matching the resolver's behavior.

Applied to all 6 agents + bmad-product-brief workflow.

## 2. Python 3.11 / OS shipping claim (customize-bmad.md)

The docs claimed "macOS 13+, Ubuntu 22.04+, Debian 12+, Fedora 37+
all ship 3.11 or newer." Inaccurate — Ubuntu 22.04 defaults `python3`
to 3.10.6 (3.11 is a separate package), and macOS doesn't really
ship Python by default anymore.

Replaced with honest guidance: check `python3 --version` and note
that macOS without Homebrew and Ubuntu 22.04 default to 3.10 or
earlier.

## 3. Autonomous mode greeting gate (bmad-product-brief)

Product-brief's activation-mode detection documents autonomous mode
as "produce complete brief without interaction" — but Step 5 greeted
unconditionally, adding conversational output before the headless
artifact. Gated the greeting on `{mode}` != `autonomous`.

## Dismissed (replied on thread)

- `.gitignore` migration from *.user.yaml to *.user.toml: YAML
  installer code was in reverted #2282, never released. No users
  affected. Same rationale as Augment's earlier thread.

Validated: 254 files, 146 refs, 0 broken. test:refs 7/7,
test:install 242/242.

docs/how-to/customize-bmad.md

@@ -215,7 +215,7 @@ python3 {project-root}/_bmad/scripts/resolve_customization.py \
   --key agent
 ```
 
-**Requirements**: Python 3.11+ (earlier versions don't include `tomllib`). No `pip install`, no `uv`, no virtualenv. macOS 13+, Ubuntu 22.04+, Debian 12+, and Fedora 37+ all ship 3.11 or newer.
+**Requirements**: Python 3.11+ (earlier versions don't include `tomllib`). No `pip install`, no `uv`, no virtualenv. Check with `python3 --version` — some common platforms (macOS without Homebrew, Ubuntu 22.04) default `python3` to 3.10 or earlier even when 3.11+ is available to install separately.
 
 `--skill` points at the skill's installed directory (where `customize.toml` lives). The skill name is derived from the directory's basename, and the script looks up `_bmad/custom/{skill-name}.toml` and `{skill-name}.user.toml` automatically.
 

src/bmm-skills/1-analysis/bmad-agent-analyst/SKILL.md

@@ -22,13 +22,13 @@ You are Mary, the Business Analyst. You bring deep expertise in market research,
 
 Run: `python3 {project-root}/_bmad/scripts/resolve_customization.py --skill {skill-root} --key agent`
 
-**If the script fails**, resolve the `agent` block yourself by merging these three files in priority order (later wins):
+**If the script fails**, resolve the `agent` block yourself by reading these three files in base → team → user order and applying the same structural merge rules as the resolver:
 
 1. `{skill-root}/customize.toml` — defaults
 2. `{project-root}/_bmad/custom/{skill-name}.toml` — team overrides
 3. `{project-root}/_bmad/custom/{skill-name}.user.toml` — personal overrides
 
-Any missing file is skipped.
+Any missing file is skipped. Scalars override, tables deep-merge, arrays of tables keyed by `code` or `id` replace matching entries and append new entries, and all other arrays append.
 
 ### Step 2: Execute Prepend Steps
 

src/bmm-skills/1-analysis/bmad-agent-tech-writer/SKILL.md

@@ -22,13 +22,13 @@ You are Paige, the Technical Writer. You transform complex concepts into accessi
 
 Run: `python3 {project-root}/_bmad/scripts/resolve_customization.py --skill {skill-root} --key agent`
 
-**If the script fails**, resolve the `agent` block yourself by merging these three files in priority order (later wins):
+**If the script fails**, resolve the `agent` block yourself by reading these three files in base → team → user order and applying the same structural merge rules as the resolver:
 
 1. `{skill-root}/customize.toml` — defaults
 2. `{project-root}/_bmad/custom/{skill-name}.toml` — team overrides
 3. `{project-root}/_bmad/custom/{skill-name}.user.toml` — personal overrides
 
-Any missing file is skipped.
+Any missing file is skipped. Scalars override, tables deep-merge, arrays of tables keyed by `code` or `id` replace matching entries and append new entries, and all other arrays append.
 
 ### Step 2: Execute Prepend Steps
 

src/bmm-skills/1-analysis/bmad-product-brief/SKILL.md

@@ -41,13 +41,13 @@ Check activation context immediately:
 
 Run: `python3 {project-root}/_bmad/scripts/resolve_customization.py --skill {skill-root} --key workflow`
 
-**If the script fails**, resolve the `workflow` block yourself by merging these three files in priority order (later wins):
+**If the script fails**, resolve the `workflow` block yourself by reading these three files in base → team → user order and applying the same structural merge rules as the resolver:
 
 1. `{skill-root}/customize.toml` — defaults
 2. `{project-root}/_bmad/custom/{skill-name}.toml` — team overrides
 3. `{project-root}/_bmad/custom/{skill-name}.user.toml` — personal overrides
 
-Any missing file is skipped.
+Any missing file is skipped. Scalars override, tables deep-merge, arrays of tables keyed by `code` or `id` replace matching entries and append new entries, and all other arrays append.
 
 ### Step 2: Execute Prepend Steps
 
@@ -68,7 +68,7 @@ Load config from `{project-root}/_bmad/bmm/config.yaml` and resolve:
 
 ### Step 5: Greet the User
 
-Greet `{user_name}` if you have not already, speaking in `{communication_language}`.
+If `{mode}` is not `autonomous`, greet `{user_name}` (if you have not already), speaking in `{communication_language}`. In autonomous mode, skip the greeting — no conversational output should precede the generated artifact.
 
 ### Step 6: Execute Append Steps
 

src/bmm-skills/2-plan-workflows/bmad-agent-pm/SKILL.md

@@ -22,13 +22,13 @@ You are John, the Product Manager. You drive PRD creation through user interview
 
 Run: `python3 {project-root}/_bmad/scripts/resolve_customization.py --skill {skill-root} --key agent`
 
-**If the script fails**, resolve the `agent` block yourself by merging these three files in priority order (later wins):
+**If the script fails**, resolve the `agent` block yourself by reading these three files in base → team → user order and applying the same structural merge rules as the resolver:
 
 1. `{skill-root}/customize.toml` — defaults
 2. `{project-root}/_bmad/custom/{skill-name}.toml` — team overrides
 3. `{project-root}/_bmad/custom/{skill-name}.user.toml` — personal overrides
 
-Any missing file is skipped.
+Any missing file is skipped. Scalars override, tables deep-merge, arrays of tables keyed by `code` or `id` replace matching entries and append new entries, and all other arrays append.
 
 ### Step 2: Execute Prepend Steps
 

src/bmm-skills/2-plan-workflows/bmad-agent-ux-designer/SKILL.md

@@ -22,13 +22,13 @@ You are Sally, the UX Designer. You translate user needs into interaction design
 
 Run: `python3 {project-root}/_bmad/scripts/resolve_customization.py --skill {skill-root} --key agent`
 
-**If the script fails**, resolve the `agent` block yourself by merging these three files in priority order (later wins):
+**If the script fails**, resolve the `agent` block yourself by reading these three files in base → team → user order and applying the same structural merge rules as the resolver:
 
 1. `{skill-root}/customize.toml` — defaults
 2. `{project-root}/_bmad/custom/{skill-name}.toml` — team overrides
 3. `{project-root}/_bmad/custom/{skill-name}.user.toml` — personal overrides
 
-Any missing file is skipped.
+Any missing file is skipped. Scalars override, tables deep-merge, arrays of tables keyed by `code` or `id` replace matching entries and append new entries, and all other arrays append.
 
 ### Step 2: Execute Prepend Steps
 

src/bmm-skills/3-solutioning/bmad-agent-architect/SKILL.md

@@ -22,13 +22,13 @@ You are Winston, the System Architect. You turn product requirements and UX into
 
 Run: `python3 {project-root}/_bmad/scripts/resolve_customization.py --skill {skill-root} --key agent`
 
-**If the script fails**, resolve the `agent` block yourself by merging these three files in priority order (later wins):
+**If the script fails**, resolve the `agent` block yourself by reading these three files in base → team → user order and applying the same structural merge rules as the resolver:
 
 1. `{skill-root}/customize.toml` — defaults
 2. `{project-root}/_bmad/custom/{skill-name}.toml` — team overrides
 3. `{project-root}/_bmad/custom/{skill-name}.user.toml` — personal overrides
 
-Any missing file is skipped.
+Any missing file is skipped. Scalars override, tables deep-merge, arrays of tables keyed by `code` or `id` replace matching entries and append new entries, and all other arrays append.
 
 ### Step 2: Execute Prepend Steps
 

src/bmm-skills/4-implementation/bmad-agent-dev/SKILL.md

@@ -22,13 +22,13 @@ You are Amelia, the Senior Software Engineer. You execute approved stories with
 
 Run: `python3 {project-root}/_bmad/scripts/resolve_customization.py --skill {skill-root} --key agent`
 
-**If the script fails**, resolve the `agent` block yourself by merging these three files in priority order (later wins):
+**If the script fails**, resolve the `agent` block yourself by reading these three files in base → team → user order and applying the same structural merge rules as the resolver:
 
 1. `{skill-root}/customize.toml` — defaults
 2. `{project-root}/_bmad/custom/{skill-name}.toml` — team overrides
 3. `{project-root}/_bmad/custom/{skill-name}.user.toml` — personal overrides
 
-Any missing file is skipped.
+Any missing file is skipped. Scalars override, tables deep-merge, arrays of tables keyed by `code` or `id` replace matching entries and append new entries, and all other arrays append.
 
 ### Step 2: Execute Prepend Steps