feat(sprint-3-4): PR lifecycle, issue linking, review ingestion, webhook idempotency, and repo trust boundary

[ashishsinghbora]

Sprint 3 & 4 Summary

• PR Lifecycle & Issue Linking (src/lib/github/webhooks.ts):
  • Auto-extracts issue references (Fixes #123, Closes #45, Resolves https://...) from PR title & description using extractIssueNumbersFromPrBody.
  • Upserts records in pull_requests mapped to the repository, project, author, and linked issue.
  • On merge: automatically transitions linked issue to state = 'closed', completes active issue claim in issue_claims, records verified contribution in contributions, and initiates contributor progression.
• Pull Request Code Reviews:
  • Ingests pull_request_review events into pull_request_reviews.
  • Reflects peer approvals (approved) and revision requests (changes_requested) directly on the PR state.
• Webhook Delivery Idempotency:
  • Uses X-GitHub-Delivery header to record deliveries in github_webhook_deliveries.
  • Automatically identifies duplicate deliveries and safely skips re-processing.
• Repository Trust Boundary:
  • Enforces repository trust checks ensuring incoming PR events originate from official, contribution-enabled repositories within the TechNexusOrg organization.
• Testing & Verification:
  • All 14 test suites and 49 unit/integration tests passing (100%).
  • 0 lint warnings, 0 type errors, clean Next.js build.