fix(undo): auto-prune snapshots to prevent unbounded growth

[boneskull]

Snapshots are now automatically pruned when counts exceed 50:

• Active snapshots pruned during Save()
• Archived snapshots pruned during Archive()

Oldest files are removed first based on timestamp in filename.

[Copilot]

Pull request overview

This pull request implements automatic pruning of undo snapshots to prevent unbounded growth in the .git/stack-undo/ directory. When snapshot counts exceed 50, the oldest files are automatically removed based on timestamp ordering in filenames.

Changes:

• Added automatic pruning logic that triggers during snapshot save and archive operations
• Implemented comprehensive tests to verify pruning behavior for both active and archived snapshots
• Updated documentation to inform users about the automatic cleanup feature

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
internal/undo/undo.go	Added pruning logic with maxActiveSnapshots and maxArchivedSnapshots constants (50 each), modified Save() and Archive() to call pruneDir(), and implemented pruneDir() function to remove oldest snapshots when limits are exceeded
internal/undo/undo_test.go	Added TestSavePrunesOldSnapshots to verify active snapshot pruning and TestArchivePrunesOldSnapshots to verify archived snapshot pruning
README.md	Updated documentation to explain that snapshots are automatically pruned to keep at most 50 pending and 50 archived, with no manual cleanup required

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