feat: recover attribution from AI checkpoints

[svarlamov]

Summary

• run AI-checkpoint mtime/ctime recovery immediately after Bash recovery and before edge/session/commit fallbacks
• query nearby candidates once for all unknown files, then rank exact-path matches before same-repository path mismatches (base commit or repo URL)
• attribute only still-unknown lines, preserving Bash and known-human attribution
• restore the original checkpoint session/tool/model metadata and emit a marked recovery metric that cannot feed back into recovery

Tests

• TestRepo coverage for ai_agent, ai_tab, zero-addition candidates, preserved known-human lines, Bash precedence, and rename/path mismatch recovery
• unit coverage for exact-path ranking and unrelated-repository rejection
• full task test, task lint, and task fmt

[svarlamov]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: recover attribution from AI checkpoints #1970 👈 (View in Graphite)
• feat: query AI checkpoint recovery candidates #1969
• main

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.