fix: candidate-extract.jq's overlap() is now a symmetric, multiplicity-free Jaccard (c9a4)

[talgolan]

Summary

• overlap() in hooks/lib/candidate-extract.jq had a numerator/denominator mismatch: the numerator counted one side's title words with multiplicity while the denominator was the deduped union, so overlap(A;B) != overlap(B;A) and short/similar titles scored above the 0.7 dedup threshold when they shouldn't (vitest/jest scored 0.909). A new dedup_key also strips the retry-burst heuristic's shared title-template suffix before scoring, so two distinct retry-bursts sharing that suffix no longer over-merge (was 0.722).
• Closes backlog item c9a4.

This was originally Task 1 of a 6-task Determinism Phase 5 plan (meta/superpowers/plans/2026-09-07-determinism-phase-5-backlog-mechanics.md). Tasks 2-6 were abandoned mid-run: they built backlog-item.sh's tag-mint/renumber mechanics and BACKLOG.md-tag-convention wiring in primer.md/end-session.md, all premised on .session-continuity/BACKLOG.md staying the backlog store — moot now that the backlog store is migrating to GitHub Issues. This commit is unrelated to backlog storage either way and stands on its own.

Full task + fix-round + re-review cycle completed via subagent-driven-development: task review approved with one Important, plan-mandated finding (a test that never actually exercised overlap()'s merge path), fixed in a follow-up round, re-review confirmed addressed with no new breakage.

Test plan

• zsh meta/superpowers/validation/2026-09-01-candidate-extract-smoke.zsh — 34/34 passing (31 pre-existing + 3 new c9a4 regression assertions: suffix-over-merge stays distinct, symmetric scoring, genuine near-duplicates still merge).
• Full meta/superpowers/validation/*.zsh suite run against this branch: all green except two pre-existing failures unrelated to this diff (stale count-pins in learnings-index-smoke.zsh/render-smoke.zsh against real BACKLOG.md/LEARNINGS.md entry counts — confirmed by diff scope that this commit never touches those files).

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