feat: add dynamic routing hints for local discovery

[Helweg]

Summary

Add plugin-side runtime routing hints that steer local conceptual discovery toward codebase-index tools without making the overall workflow semantic-first.

Changes

• add a dynamic routing hint controller that classifies turns into conceptual discovery, definition lookup, exact identifier, external, direct-path, or other requests
• gate runtime hint injection behind the new search.routingHints config flag and route definition-style queries toward implementation_lookup
• fix plugin runtime wiring so routing hints, auto-indexing, and file watching use the same shared indexer instance as the tool surface
• harden exact-identifier routing by removing stateful backticked-query detection and adding a repeated-query regression test
• document the new routing behavior, config option, and definition-first workflow in the README

Testing

How were these changes tested?

• Unit tests added/updated
• Manual testing performed
• Build passes (npm run build)
• Typecheck passes (npm run typecheck)
• Tests pass (npm run test:run)
• Lint passes (npm run lint)

Additional verification performed:

• npx vitest run tests/routing-hints.test.ts tests/config.test.ts
• npx vitest run tests/mcp-server.test.ts tests/commands.test.ts tests/search-integration.test.ts
• npx vitest run tests/routing-hints.test.ts tests/config.test.ts tests/search-integration.test.ts tests/mcp-server.test.ts

Release Labels

• Added at least one release category label (feature, bug, performance, documentation, dependencies, refactor, test, chore, or skip-changelog)
• Added at most one semver label (semver:major, semver:minor, semver:patch) when needed

Related Issues

Fixes #(issue number)