feat: prompt before cleaning up stale sync state

[javoire]

When running stack sync and there's leftover state from a previous interrupted sync, the command now prompts the user to confirm before starting fresh.

Previously it would just print a warning and automatically continue, which could be surprising.

Now the user sees:

Warning: found state from a previous interrupted sync
If you resolved rebase conflicts, run 'stack sync --resume'

Start fresh? [y/N]

• Default is No (pressing Enter aborts)
• Typing y or yes cleans up and continues
• Aborted state shows helpful message about --resume and --abort options

[javoire]

🎉 This PR is included in version 1.5.0 🎉

The release is available on GitHub release

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