fix(core): align grep behavior and guidance

[rekram1-node]

Summary

• require external-directory approval for Grep paths outside the active location
• surface actionable invalid-regex errors and accurate no-match output
• clarify the Grep description and parameter descriptions
• align formatter naming with Glob

Testing

• bun test test/tool-search.test.ts from packages/core
• bun typecheck from packages/core
• git diff --check