feat: add --resume flag to stack sync for rebase conflict recovery

[javoire]

When stack sync encounters a rebase conflict, the stash is now preserved and users are instructed to run stack sync --resume after resolving conflicts.

Changes

• Add --resume flag to continue sync after resolving rebase conflicts
• Save sync state (stashed status, original branch) to git config
• Don't pop stash on rebase conflicts - wait for --resume
• Improved error message with step-by-step instructions
• Validation for orphaned sync state and missing resume state

Workflow

1. User runs stack sync with uncommitted changes
2. Changes are stashed
3. Rebase conflict occurs
4. User resolves conflicts, runs git rebase --continue
5. User runs stack sync --resume
6. Sync completes and stash is popped

[javoire]

🎉 This PR is included in version 1.2.0 🎉

The release is available on GitHub release

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