chore(tools): remove Claude Code-specific sections from skill cheatsheet

Keep cheatsheet focused on the Agent Skills open standard only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tools/skill-validator.md

@@ -325,7 +325,7 @@ If zero findings: report "All {N} rules passed. No findings." and list all passe
 
 ## Skill Spec Cheatsheet
 
-Quick-reference for the Agent Skills open standard and Claude Code extensions.
+Quick-reference for the Agent Skills open standard.
 For the full standard, see: [Agent Skills specification](https://agentskills.io/specification)
 
 ### Structure
@@ -351,48 +351,6 @@ For the full standard, see: [Agent Skills specification](https://agentskills.io/
 - Skills are filesystem-based directories, not API payloads — Claude reads them via bash/file tools
 - Keep SKILL.md focused; offload detailed reference to separate files
 
-### Cross-platform portability
-- Same SKILL.md format works on Claude Code, Claude API, Claude.ai, Agent SDK
-- Runtime differs per surface: Code has full network access, API has none, claude.ai varies
-- Skills don't sync across surfaces — deploy separately to each
-
-### Extra frontmatter fields (Claude Code only)
-- `disable-model-invocation`: `true` = only user can invoke via `/name`
-- `user-invocable`: `false` = hidden from `/` menu, only Claude auto-loads it
-- `allowed-tools`: restrict tools available during skill execution
-- `model`: override model for this skill
-- `context`: `fork` runs skill in isolated subagent
-- `agent`: which subagent type for `context: fork` — `Explore`, `Plan`, `general-purpose`, or custom
-- `hooks`: skill-scoped lifecycle hooks
-- `argument-hint`: autocomplete hint like `[issue-number]`
-
-### String substitutions
-- `$ARGUMENTS` / `$ARGUMENTS[N]` / `$N` — argument placeholders
-- `${CLAUDE_SESSION_ID}` — current session ID
-- If `$ARGUMENTS` absent from content, args appended as `ARGUMENTS: <value>`
-
-### Dynamic context injection
-- `` !`command` `` syntax runs shell commands at load time, output replaces placeholder
-- Runs before Claude sees content — pure preprocessing
-
-### Subagent execution
-- `context: fork` = skill runs in isolated context without conversation history
-- Only useful for skills with explicit task instructions, not pure guidelines
-- `agent` field picks execution environment; defaults to `general-purpose`
-
-### Invocation control matrix
-| Frontmatter | User invokes | Claude invokes | Description in context |
-|---|---|---|---|
-| (default) | yes | yes | yes |
-| `disable-model-invocation: true` | yes | no | no |
-| `user-invocable: false` | no | yes | yes |
-
-### Context budget
-- Skill descriptions budget = 2% of context window, fallback 16k chars
-- Override with `SLASH_COMMAND_TOOL_CHAR_BUDGET` env var
-
 ### Practical tips
 - Keep SKILL.md under 500 lines
 - `description` drives auto-discovery — use keywords users would naturally say
-- Include "ultrathink" in skill content to enable extended thinking
-- `.claude/commands/` files still work but skills take precedence on name collision