Strengthen snapshot navigation metadata

[itsflaid]

Summary

• add top function/code symbol metadata to snapshot fileIndex entries
• add feature entry points, business flows, request flows, onboarding path, and change impact metadata
• add snapshot-first agent instructions and a stronger generated DEVMAP.md agent contract
• detect Groq external service from source-level API signals

Verification

• pnpm --filter devmap exec tsx --test test/analyzers.test.ts test/context-builder.test.ts test/json-output.test.ts test/ai-client.test.ts
• pnpm --filter devmap exec tsx --test test/analyzers.test.ts test/init-and-errors.test.ts test/json-output.test.ts
• pnpm test:cli
• pnpm build:cli
• git diff --check