feat: prevent duplicate review comments with content-based dedup

[makinosp]

Summary

Prevents the review agent from posting the same review comment repeatedly, even when the LLM reports the same concern on a different line.

Previously, incremental mode only skipped comments whose line range overlapped a previously-posted bot comment. With content-based dedup, a comment is also skipped when its wording matches a bot comment already posted on the same path, regardless of line.

Changes

• Content-based deduplication (src/domain/post-comments.ts)
  • normalizeContent — strips the summary marker, Markdown decoration, collapses whitespace, lowercases.
  • jaccardSimilarity — token-set Jaccard coefficient.
  • isDuplicateContent — normalized exact match or Jaccard similarity at-or-above threshold.
  • isDuplicateComment — combines the existing line-overlap rule with the content rule (OR-ed), same path + RIGHT-side history only.
• incremental is now enabled by default (actions/review/action.yml, README) — default changed false → true.
• New action inputs
  • content-based-deduplication (default true) — enable/disable the content rule.
  • content-similarity-threshold (default 0.8) — Jaccard threshold in (0, 1].
• CLI plumbing (src/cli.ts) — REVIEW_CONTENT_BASED_DEDUPLICATION, REVIEW_CONTENT_SIMILARITY_THRESHOLD env vars wired through ReviewCliConfig → ReviewOptions → postReview.
• Tests — 24 new tests covering normalizeContent, jaccardSimilarity, resolveContentThreshold, isDuplicateContent, isDuplicateComment, and postReview content-based dedup integration.

Validation

• pnpm typecheck ✅
• pnpm test — 215/215 ✅
• pnpm lint ✅

Notes

This branch is based on main and contains only the dedup feature (no formatting/refactor changes — those live in PR #1).