feat: implement Issue Marketplace and deterministic recommendation engine (Phase 3)

[ashishsinghbora]

What changed

• Built interactive Issue Marketplace (/issues) with multi-faceted filtering by difficulty, language, technology, and 'good first issue' flag.
• Created deterministic, explainable issue recommendation engine (src/lib/recommendations/engine.ts) that scores alignment with contributor skills, experience level, and preferred languages.
• Added 'Recommended Contributions For You' section on the Contributor Dashboard with transparent match scores and explainable rationale.
• Implemented GitHub issue metadata extractor and label parser (src/lib/github/issues.ts).
• Added secure issue synchronization endpoint at /api/sync/issues.
• Enriched Project Directory (/projects) with live open issue counts and direct marketplace exploration links.
• Added comprehensive unit tests for label parsing and recommendation ranking.

Verification

• Vitest: 9/9 test suites, 28/28 tests passing.
• TypeScript tsc --noEmit: 0 errors.
• ESLint: 0 errors, 0 warnings.
• Production build: all 13 routes compiled and optimized.