feat(source-loader): GitHub Issues loader

[mroops0111]

Closes #28.

Summary

• New @braidhq/source-loader-github plugin: ingests issues from a GitHub repo via REST, renders each as <destination>/issues/<number>.md with deterministic YAML frontmatter + body + sorted ## Comments.
• sync keeps a since cursor in .braid-github-cursor.json and uses content-hash-equal writeIfChanged so untouched issues stay byte-identical across syncs (mtime preserved → downstream sha fingerprints don't churn).
• Pagination via Link: …; rel="next". Pull requests filtered out by default (includePullRequests: false).
• Token via \${GH_TOKEN} env interpolation; anonymous mode supported but rate-limited.
• Registered in composeFs only when GH_TOKEN is set — anonymous GitHub gives 60 req/h, which won't survive a realistic sync, so skipping registration surfaces the misconfig at workspace load time rather than mid-batch.

Test plan

• Unit tests (6 pass): N issues → N files, PR filter, sorted comments, Link pagination, sync cursor + byte-stability, Authorization header from env
• pnpm typecheck green
• pnpm lint:fix clean
• End-to-end dogfood: register braid workspace with kind: 'github' source pointing at this repo; verify sync lands markdown under sources/<id>/issues/ and /braid-extract produces sensible proposals