feat: add CodeRabbit AI code review integration

- Add .coderabbit.yaml with minimal config and path instructions
- Exclude node_modules from review scope
- Document pilot research and conclusions in docs/planning/

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.coderabbit.yaml

@@ -0,0 +1,36 @@
+# yaml-language-server: $schema=https://coderabbit.ai/integrations/schema.v2.json
+
+language: "en-US"
+early_access: true
+reviews:
+  profile: chill
+  high_level_summary: true
+  request_changes_workflow: false
+  review_status: true
+  collapse_walkthrough: false
+  poem: false
+  auto_review:
+    enabled: false # must be manually triggered with @coderabbit review
+    drafts: true # Can review drafts. Since it's manually triggered, it's fine.
+    auto_incremental_review: false # always review the whole PR, not just new commits
+    base_branches:
+      - main
+  path_filters:
+    - "!**/node_modules/**"
+  path_instructions:
+    - path: "**/*"
+      instructions: |
+        Focus on inconsistencies, contradictions, edge cases and serious issues.
+        Avoid commenting on minor issues such as linting, formatting and style issues.
+        When providing code suggestions, use GitHub's suggestion format:
+        ```suggestion
+        <code changes>
+        ```
+    - path: "**/*.js"
+      instructions: |
+        CLI tooling code. Check for: missing error handling on fs operations,
+        path.join vs string concatenation, proper cleanup in error paths.
+        Flag any process.exit() without error message.
+chat:
+  auto_reply: true # Response to mentions in comments, a la @coderabbit review
+