feat: checklist engine and HTML-comment metadata

[shouze]

Summary

Pure-function core logic: checklist building/parsing/diffing and HTML-comment metadata embedding.

Tasks

• src/core/checklist.ts
  • BODY_LIMIT = 65_000
  • buildChecklist(results) → markdown checklist lines
  • parseChecklist(body) → ChecklistItem[]
  • diffChecklist(old, new) → DiffResult keyed on repo:path:line:text
  • applyDiff(body, diff) → updated body
  • buildSummaryBlock / updateSummaryBlock
  • checkBodyLength
• src/core/metadata.ts
  • embedMetadata(body, meta) — HTML comment at end of body
  • extractMetadata(body) → EpicMetadata | null
  • updateMetadata(body, patch) — partial update
  • stripMetadata(body) — remove for display

Metadata format

<!-- github-issue-ops:metadata
{"version":1,"replayCommand":"...","createdAt":"...","config":{...}}
-->

Acceptance criteria

• Unit tests in src/core/checklist.test.ts and src/core/metadata.test.ts pass
• Round-trip: extractMetadata(embedMetadata(body, meta)) equals meta
• Diff is idempotent: re-applying the same results produces no new changes