test(mcp): cover remaining CLI gaps

[bntvllnt]

Summary

• replace the remaining explicit todo tests with real MCP contract tests
• add shared in-memory MCP fixture helper for tests
• verify search result grouping/symbol locations/nextSteps, symbol_context metadata/nextSteps, and detect_changes no-git error behavior
• suppress raw git diff usage output on structured detect_changes failures

Verification

• pnpm lint
• pnpm typecheck
• pnpm test
• rg todo/skip scan: no matches

Release

• Canary mode only. Do not run the manual stable release workflow.