fix(den-api): invalidate GitHub discovery cache by head sha

[kgarg2468]

Summary

• fetch the current GitHub branch head SHA as part of discovery context
• require cached discovery entries to match sourceRevisionRef before reuse
• add focused tests for cache freshness and the head SHA helper

Fixes #1871.

Tests

• bun test ee/apps/den-api/test/github-discovery.test.ts ee/apps/den-api/test/github-connector-app.test.ts
• pnpm --filter @openwork-ee/den-api exec tsc -p tsconfig.json --noEmit --pretty false
• git diff --check

[vercel]

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Project	Deployment	Actions	Updated (UTC)
openwork-landing	Ready	Preview, Comment, Open in v0	May 22, 2026 2:34am

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