Replace unreliable filesystem ctime/mtime with git log timestamps.
A single batch `git log` walks the full commit history to extract
created_at (oldest commit) and modified_at (newest commit) for each
.md file. Falls back to filesystem dates for non-git vaults and
uncommitted files. Cache version bumped to 10 for full rescan.
ADR 0039 documents the decision.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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