fix(pr-management-triage): dedupe paginated PRs

[awishere]

Summary

• Deduplicate PRs by number after the full pagination loop.
• Keep the final occurrence so an updated PR uses its freshest snapshot and correct position.
• Add regression coverage for a PR moving between pages and for an already-unique result set.

Type of change

• Skill change (skills/<name>/) — eval fixtures updated below
• Tool / bridge contract (tools/<system>/*.md)
• Python package
• Documentation
• CI / dev loop

Test plan

• prek run --all-files passes
• All 35 pr-management-triage eval fixtures assemble successfully
• Both pagination-dedup cases pass in exact automated mode
• All 150 skill-eval framework tests pass
• Regression fixtures are included

RFC-AI-0004 compliance

• Vendor neutrality — no provider-specific behavior added

Linked issues

Closes #77

Generative AI disclosure

Codex was used while preparing this change. The contributor reviewed the diff and test results before submission.