enhance(backlog-triage): add stale signals for merged PRs and closed duplicates

[sungjunlee]

Context

#73 added the collector-side data needed for stronger obsolete-candidate detection: per-issue closing_prs and optional top-level closed_issues. triage-stale.js still uses only inactivity and explicit labels, so the richer snapshot fields are not yet used for stale decisions.

Desired change

Add conservative stale/obsolete candidates based on snapshot-v2 fields.

Candidate signals:

• open issue has a merged closing PR in closing_prs
• open issue appears to duplicate a recently closed issue from closed_issues

Acceptance Criteria

• Merged closing-PR candidates require closing_prs evidence and include PR number/url/mergedAt in candidate evidence.
• Duplicate-of-closed candidates run only when top-level closed_issues is present.
• Duplicate-of-closed matching is conservative and tested against false positives.
• Missing optional fields degrade cleanly without warnings or undefined-field behavior.
• Stale docs and tests cover enabled and absent-field paths.

[coderabbitai]

🔗 Related PRs

#74 - feat(backlog-triage): triage report renderer (#64) [merged]
#80 - fix(backlog-triage): exclude progress issues from triage snapshots [merged]
#82 - feat(backlog-triage): add snapshot v2 collector [merged]
#174 - feat(backlog-triage): add spec-aware Decision Review [merged]

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