feat: schedule periodic full repacks via job scheduler

[alecthomas]

Summary

• Adds periodic full repack (git repack -adb --write-midx --write-bitmap-index) for mirror repos via the existing job scheduler
• Configurable via repack-interval on the git strategy (default 0 = disabled)
• Scheduled for both newly cloned and existing-on-startup repos, serialized per-repo with fetch/clone/snapshot jobs

Closes #126

Test plan

• Unit test for Repository.Repack() verifying pack file and multi-pack-index creation
• Strategy tests for enabled/disabled config and scheduling with existing repos on disk
• All existing tests pass
• Linter clean

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