Add provenance tracking and source repo mode

[HartBrook]

Summary

• Provenance tracking: Merged CLAUDE.md now includes <!-- staghorn:source:LAYER --> comments marking content origin, enabling tooling to understand which layer contributed each section
• Source repo detection: .staghorn/source.yaml marks team/community repos for special handling
• Source repo mode: When inside a source repo, commands (edit, info, optimize, eval) operate on local files instead of cache for seamless development workflow
• Provenance-aware optimization: stag optimize --apply now works with merged configs, splitting content back to source layers
• Flatter language structure: Language sections are now top-level H2 headers instead of nested

Test plan

• Run go test ./... to verify all tests pass
• Test stag team init creates .staghorn/source.yaml
• Test stag sync produces provenance markers in output
• Test stag optimize --apply splits merged content by provenance
• Test source repo mode: stag edit team works inside a source repo