Move contributor scoring into GitHub app

[homanp]

Summary

• Move contributor trust scoring into the GitHub app by porting local identity, behavior, and content scoring.
• Replace the external contributor API call with GitHub REST/GraphQL signal collection plus short-TTL SQLite caching.
• Remove the contributor graph dimension from result types, comments, and tests while leaving PR scan API usage unchanged.

Test plan

• npm run typecheck
• npm test
• npm run build
• Local health check: curl http://localhost:3000/health
• Local contributor scan for homanp using the app installation token