feat: add completeness pass and review quality improvements to pr-review

[k-rister]

Summary

• Add Pass 6 (Completeness) to catch files that should be in the diff but aren't — CLAUDE.md co-change rules, stale doc cross-references, and deduplication against earlier passes
• Add git history context step — git blame and recent merged PRs on the same files to understand why code was written a certain way
• Add explicit False Positive Exclusions list — pre-existing issues, unmodified lines, linter territory, pedantic nitpicks, intentional behavior changes
• Add Confidence Check step — self-verification before finalizing the report to drop low-confidence findings

Motivated by crucible PR#645 review cycle where all three findings from the second review pass were missing co-changes in files outside the diff (bin/_help, docs/how-services-work.md, schema/services.json). The completeness pass would have caught all three in a single review. The other improvements are adopted from methodology patterns in Claude Code's built-in /code-review command.

Test plan

• Run /crucible-tools:pr-review against a PR with a known missing co-change and verify Pass 6 catches it
• Run against a clean PR and verify no false positives from the new passes
• Verify git history context doesn't produce noise on files with long blame histories

🤖 Generated with Claude Code

[atheurer]

This looks like an outstanding improvement. Adding the completeness check, git history/blame context, and clear false-positive exclusions makes the review skill incredibly robust and prevents siloed findings. Approve!