fix: prevent duplicate PR comments via similarity-based deduplication (issue #1495)

[konard]

Summary

Fixes #1495 — Multiple repeated comments were posted after ready to merge on link-foundation/sandbox#73. After an AI work session completed, 4+ duplicate "Validation Complete" comments were posted within a 37-second window.

Root Cause

The hive-mind system had no deduplication mechanism for:

1. AI agent-generated validation/summary comments
2. Cross-session duplicate comments (different processes posting to the same PR)
3. Post-session comment leaks from concurrent processes

Changes

• New comment-dedup.lib.mjs: Text normalization, overlap-coefficient similarity scoring, and deduplication check before posting comments
  • normalizeCommentForComparison() — strips markdown, timestamps, emoji, table formatting
  • computeSimilarity() — overlap coefficient on keyword sets
  • computeCommentSimilarity() — combines header matching with word overlap for better accuracy
  • checkForRecentSimilarComment() — checks recent PR comments within a time window
  • postCommentWithDedup() — drop-in replacement for gh pr comment with built-in dedup
• Integrated dedup into solve.auto-merge.lib.mjs: "Ready to merge" and "Auto-merged" comments now use postCommentWithDedup() with 10-minute window and 0.7 similarity threshold
• AI agent prompt enhancement: Added "Comment deduplication" section to all 4 prompt templates (claude, codex, opencode, agent) instructing the AI to check for existing similar comments before posting
• Case study documentation: Timeline reconstruction and root cause analysis in docs/case-studies/issue-1495/
• Changeset: Patch version bump

Test Plan

• 26 unit tests in tests/comment-dedup.test.mjs — all passing
• Tests cover: text normalization, similarity scoring, header extraction, real-world duplicate detection from the PR We should be specific, and tell the model actual mergeStateStatus #73 incident
• Prettier formatting verified
• CI checks pass after push

[konard]

🤖 Solution Draft Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Public pricing estimate: $15.698279
• Calculated by Anthropic: $10.339753 USD
• Difference: $-5.358526 (-34.13%)

🤖 Models used:

• Tool: Anthropic Claude Code
• Requested: opus
• Main model: Claude Opus 4.6 (claude-opus-4-6)
• Additional models:
  • Claude Haiku 4.5 (claude-haiku-4-5-20251001)

📎 Log file uploaded as Gist (3579KB)

• View complete solution draft log

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Now working session is ended, feel free to review and add any feedback on the solution draft.

[konard]

🔄 Auto-restart 1/3

Detected uncommitted changes from previous run. Starting new session to review and commit or discard them.

Uncommitted files:

?? gist-log.txt
?? pr73-comments.txt
?? pr73-issue-comments.json

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Auto-restart will stop after changes are committed or discarded, or after 2 more iterations. Please wait until working session will end and give your feedback.

[konard]

🔄 Auto-restart 1/3 Log

This log file contains the complete execution trace of the AI solution draft process.

💰 Cost estimation:

• Public pricing estimate: $3.911514
• Calculated by Anthropic: $2.240721 USD
• Difference: $-1.670793 (-42.71%)

🤖 Models used:

• Tool: Anthropic Claude Code
• Requested: opus
• Model: Claude Opus 4.6 (claude-opus-4-6)

📎 Log file uploaded as Gist (4547KB)

• View complete solution draft log

---

Now working session is ended, feel free to review and add any feedback on the solution draft.

[konard]

✅ Ready to merge

This pull request is now ready to be merged:

• All CI checks have passed
• No merge conflicts
• No pending changes

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Monitored by hive-mind with --auto-restart-until-mergeable flag