test(compare): add E2E tests for git baseline comparison

[AmiteK23]

Description

Add end-to-end tests for the --baseline git:<ref> comparison feature.

Validates comparisons against branches, tags, commit hashes, and relative refs using real git repositories.
Also verifies worktree cleanup and error handling.

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Type of Change

• Bug fix
• Feature
• Breaking change
• Docs
• Refactor
• CI / Tests
• Performance
• Other (please describe):

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Architectural Impact

• No contract/output changes
• Contract/output changed (describe below)

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Testing

• Tested locally
• Tests added/updated
• All tests pass

New E2E tests cover:

• Git baseline comparison using branches, tags, commit hashes, and relative refs
• Drift detection after structural changes
• Git-ignored file filtering
• Error handling (invalid refs, non-git repositories)
• Cleanup verification for temporary git worktrees

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Related Issues

None.

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Checklist

• Target branch is main
• Self-reviewed
• Docs updated (if needed)
• Changelog updated (if needed)

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Additional Notes

These tests use temporary git repositories created during the test run to simulate real comparison scenarios and validate the full CLI workflow.