enhance(backlog-triage): emit comment and closing-PR relationship signals

[sungjunlee]

Context

#73 added the snapshot-v2 collector fields needed for richer relationship analysis: per-issue closing_prs is present by default and comments is available through --with-comments. The collector is ready, but triage-relate.js still emits only body-based mentions/blocks/depends-on and title duplicate candidates.

Desired change

Teach triage-relate.js to use snapshot-v2 fields conservatively.

Suggested signals:

• comment-based mentions when comments is present
• a closing-PR relationship signal when closing_prs contains merged PR metadata

Acceptance Criteria

• Comment mention scanning runs only when issue comments arrays are present.
• Comment-derived edges carry evidence that identifies comment source separately from issue body evidence.
• Closing-PR edges use closing_prs and do not imply an automatic close recommendation by themselves.
• Missing optional fields degrade cleanly without warnings or undefined-field behavior.
• Relationship docs and tests cover both 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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