fix(snapshot): optimize cleanup with stale pack removal and conditional gc

[riatzukiza]

Issue for this PR

Closes #74

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This phase replaces unconditional heavy cleanup with threshold-driven maintenance. It removes stale tmp_pack_* artifacts under lock, runs gc --auto only when loose objects are high, and only escalates to prune when pack count remains elevated.

How did you verify your code works?

• Ran bun test test/snapshot/snapshot.test.ts from packages/opencode.
• Verified stale tmp_pack_* cleanup and low-loose-object behavior tests pass.

Screenshots / recordings

N/A (non-UI change).

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

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