Spool sandboxed checkpoints for daemon import

[svarlamov]

Summary

• Route sandbox and daemon-unavailable checkpoints through a metadata-only fallback on macOS and Linux.
• Persist private, atomic records under /tmp/git-ai-sandboxed-checkpoints without running normal file or Bash checkpoint logic.
• Import up to 1,000 records per sequential daemon batch, every three seconds after the prior batch finishes.
• Store file_edit_sandboxed and bash_sandboxed checkpoint metrics while retaining failed records for retry.

Testing

• task build
• task fmt
• task lint
• task test

[svarlamov]

• Recover attribution from sandboxed checkpoints #1914
• Spool sandboxed checkpoints for daemon import #1913 👈 (View in Graphite)
• Refuse daemon startup in agent sandboxes #1912
• main

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