feat(agents): add learnings-researcher for institutional knowledge (#106)

* fix(compound-docs): correct severity enum in validation example

The example error message referenced [critical, moderate, minor] but
the yaml-schema.md defines severity as [critical, high, medium, low].

* refactor(agents): standardize search tool recommendations

- Update repo-research-analyst to recommend built-in Grep tool instead
  of CLI rg (Grep uses ripgrep under the hood)
- Add TypeScript example for ast-grep alongside Ruby
- Update pattern-recognition-specialist to use built-in Grep tool
- Keep ast-grep for AST-based structural matching (language-specific)

* feat(agents): add learnings-researcher for institutional knowledge

Add new research agent that efficiently searches docs/solutions/ for
relevant past solutions before implementing features or fixing bugs.

Key features:
- Grep-first filtering strategy for efficiency with 100+ files
- Category-based narrowing to reduce search scope
- Parallel Grep calls with synonym support (OR patterns)
- Frontmatter-only reads before full document reads
- Always checks critical patterns file
- Uses haiku model for speed (structured task, recipe-based)

Integrates with /workflows:plan to run in parallel with
repo-research-analyst during local research phase.

Closes the loop on compound engineering: solutions documented via
/workflows:compound are now discoverable during planning.

plugins/compound-engineering/agents/research/repo-research-analyst.md

@@ -96,10 +96,11 @@ Structure your findings as:
 
 **Search Strategies:**
 
-When using search tools:
-- For Ruby code patterns: `ast-grep --lang ruby -p 'pattern'`
-- For general text search: `rg -i 'search term' --type md`
-- For file discovery: `find . -name 'pattern' -type f`
+Use the built-in tools for efficient searching:
+- **Grep tool**: For text/code pattern searches with regex support (uses ripgrep under the hood)
+- **Glob tool**: For file discovery by pattern (e.g., `**/*.md`, `**/CLAUDE.md`)
+- **Read tool**: For reading file contents once located
+- For AST-based code patterns: `ast-grep --lang ruby -p 'pattern'` or `ast-grep --lang typescript -p 'pattern'`
 - Check multiple variations of common file names
 
 **Important Considerations:**