feat: Archive comparison and changelog

[Snider]

Summary

Generate human-readable changelog between archive versions.

Use Case

When syncing periodically, show what changed in a readable format.

Commands

# Generate changelog
borg changelog old.stim new.stim

# Output as markdown
borg changelog old.stim new.stim --format markdown > CHANGELOG.md

# Compare with remote source
borg changelog archive.stim --source github:org/repo

Output

# Changes: 2024-01-10 → 2024-01-15

## Added (12 files)
- src/features/new-api.go
- docs/api-guide.md
- tests/api_test.go

## Modified (5 files)
- README.md (+15 -3 lines)
- go.mod (dependency updates)
- src/main.go (refactored)

## Removed (2 files)
- deprecated/old-handler.go
- tests/legacy_test.go

## Stats
- 19 files changed
- +523 insertions, -89 deletions

Acceptance Criteria

• Detect added/modified/removed files
• Line-level diff stats
• Markdown output format
• JSON output for processing
• Commit message extraction

[google-labs-jules]

Jules is on it. When finished, you will see another comment and be able to review a PR.

[google-labs-jules]

Ready for a review! A PR has been created.