feat(mcp): restrict grep_app tools to librarian agent only (#395)

* feat(mcp): restrict grep_app tools to librarian agent only

Reduces token usage by disabling grep_app MCP tools globally and enabling them only for the librarian agent, which uses them for GitHub code search during documentation lookups.

Changes:
- Add grep_app_* tool disable globally in config.tools
- Add grep_app_* tool enable for librarian agent
- Remove grep_app references from explore agent prompt (no access)

Closes #394

* chore: changes by sisyphus-dev-ai

---------

Co-authored-by: sisyphus-dev-ai <sisyphus-dev-ai@users.noreply.github.com>

src/agents/explore.ts

@@ -108,18 +108,8 @@ Use the right tool for the job:
 - **Text patterns** (strings, comments, logs): grep
 - **File patterns** (find by name/extension): glob
 - **History/evolution** (when added, who changed): git commands
-- **External examples** (how others implement): grep_app
 
-### grep_app Strategy
-
-grep_app searches millions of public GitHub repos instantly — use it for external patterns and examples.
-
-**Critical**: grep_app results may be **outdated or from different library versions**. Always:
-1. Start with grep_app for broad discovery
-2. Launch multiple grep_app calls with query variations in parallel
-3. **Cross-validate with local tools** (grep, ast_grep_search, LSP) before trusting results
-
-Flood with parallel calls. Trust only cross-validated results.`,
+Flood with parallel calls. Cross-validate findings across multiple tools.`,
   }
 }