feat: add P1 PR consolidation analysis

[Wibias]

Summary

• add deterministic clustering for potentially competing PR implementations
• treat a shared durable work-item identity as related-work evidence, not automatic replacement proof
• promote same-work-item pairs to high confidence only when they also have substantial non-noise implementation overlap
• allow substantial non-noise changed-file overlap without a shared work item as medium-confidence evidence
• deliberately leave canonicalPr unset during analysis
• keep analysis read-only and reuse the existing supersede-pr workflow for any later closing/comment mutations
• expose competing-PR analysis in SKILL.md discovery metadata

Hardening found during self-review

• a selected canonical PR must have direct supersede-grade implementation-overlap evidence to every PR it would supersede
• transitive cluster chains such as A-B-C cannot let A supersede C without direct A-C evidence
• a shared tracker/work-item key alone can form a candidate cluster but cannot produce a supersede plan, because one ticket may intentionally use complementary PRs
• child routing keeps feat: add P1 work-item delivery orchestration #284's work-item publication and tracker-status fixes instead of reverting them during stack updates

Stack

• depends on feat: add P1 work-item delivery orchestration #284
• feat: add P1 work-item delivery orchestration #284 depends on feat: add P0 open-work and publication safety #283

Validation

• focused contracts cover related-work clusters, supersede-grade overlap, file-overlap thresholds, cross-base isolation, noise-only overlap, canonical selection, transitive-cluster rejection, same-key-without-overlap rejection, manufactured-cluster rejection, routing, mode, and controller phases
• repository-wide local suite is intentionally left to the user's local run; GitHub Actions is not used as the current gate

Provenance

The clustering direction was informed by OutThisLife/brooklyn-skills pr-triage and redesigned to compose with GitHub Delivery's existing supersede machinery.

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