feat(backlog-triage): relationship detection (mentions, blocks, duplicates)

[sungjunlee]

Part of #59 (epic). Depends on #61 (snapshot contract).

Scope

Detect relationships between open issues. Consumes the snapshot file produced by triage-collect.js; does NOT re-fetch. Output feeds the triage report's Relationships section.

Signals (rule-based, MVP)

• Mentions (body only) — #N references in issue bodies. Comments are deferred to enhance(backlog-triage): snapshot v2 schema for closing PRs, comments, and closed-issue scan #73 — the snapshot does not yet carry them.
• Blocks / depends-on — keyword scan in body: "blocks #N", "blocked by #N", "depends on #N", "closes #N"
• Duplicate candidates — title similarity (normalized Jaccard on token sets) above a configurable threshold
• PR linkage (deferred to enhance(backlog-triage): snapshot v2 schema for closing PRs, comments, and closed-issue scan #73) — surfacing already-merged PRs referencing open issues requires the closing_prs snapshot extension. Not in this MVP.

Config

Duplicate threshold and signal weightings live in backlog/triage-config.yml (defined in #61). references/classification.md documents the schema.

Acceptance Criteria

• scripts/triage-relate.js --snapshot <path-to-snapshot.json> [--json]
• No re-fetching — reads snapshot only. Errors clearly if --snapshot missing or malformed
• Emits edges: {from: N, to: M, kind: 'mentions'|'blocks'|'depends-on'|'duplicate-candidate', confidence, evidence}
• Evidence includes the snippet that triggered the match (PR URL evidence lands with enhance(backlog-triage): snapshot v2 schema for closing PRs, comments, and closed-issue scan #73 + follow-up)
• Duplicate threshold configurable via backlog/triage-config.yml
• No mutations; pure read
• Node test with fixture snapshot covering each implemented edge kind
• references/relationships.md documents the deferred PR linkage signal and points at enhance(backlog-triage): snapshot v2 schema for closing PRs, comments, and closed-issue scan #73

Deferred to follow-up

• comments-based mention scan (requires enhance(backlog-triage): snapshot v2 schema for closing PRs, comments, and closed-issue scan #73 snapshot extension)
• PR linkage edge kind (requires enhance(backlog-triage): snapshot v2 schema for closing PRs, comments, and closed-issue scan #73 closing_prs snapshot extension)

Both are tracked at #73. Re-scope came from review of initial dispatch (PR #72).