🧹 Fix indiscriminate branch deletion in cleanup logic

[Cylae]

🎯 What

Fixes the cleanup logic in script.ps1 to track branches checked out during PR resolution and only delete those branches instead of deleting all local branches indiscriminately.

💡 Why

The previous logic iterated through git branch and deleted all local branches except the base branch. This posed a risk of deleting user's un-related local branches that existed before running the script. By tracking checked out branches in a HashSet[string], the script now selectively cleans up only the PR branches it explicitly checked out.

✅ Verification

• Checked script syntax with pwsh.
• Executed script.ps1 against a temporary Git repo with a mock gh CLI. Confirmed that PR branches checked out during resolution were cleaned up while other local branches were preserved.
• Code review requested and approved.

✨ Result

Local branch cleanup is now safe and targeted to only script-created/checked-out PR branches.

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PR created automatically by Jules for task 1244228609587489830 started by @Cylae

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