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/skills/compound-docs/SKILL.md

@@ -159,7 +159,7 @@ Load `schema.yaml` and classify the problem against the enum values defined in [
 
 Errors:
 - problem_type: must be one of schema enums, got "compilation_error"
-- severity: must be one of [critical, moderate, minor], got "high"
+- severity: must be one of [critical, high, medium, low], got "invalid"
 - symptoms: must be array with 1-5 items, got string
 
 Please provide corrected values.