fix: improve sync robustness for remote branch detection and PR handling

[javoire]

Fixes several edge cases in stack sync:

• Sentinel error for cleaner output: Adds errAlreadyPrinted to avoid duplicate error messages when detailed instructions are already shown
• Better remote branch detection: Now detects remote branches via PR cache even when local tracking ref is missing
• Auto-fetch missing refs: Fetches remote tracking refs on-demand when they're missing but branch has a PR
• Fix FetchBranch refspec: Uses proper refspec to ensure tracking refs are created/updated (not just FETCH_HEAD)
• Prefer OPEN PRs: When multiple PRs exist for the same branch, prefers OPEN ones over closed/merged
• Cleaner error messages: Consolidated divergence error handling with better recovery instructions

[javoire]

🎉 This PR is included in version 1.2.2 🎉

The release is available on GitHub release

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