⚡ Bolt: [performance improvement] Batch local git branch deletions

[Cylae]

💡 What:
Refactored the local git branch cleanup loop in script.ps1. Replaced iterating over branches and spawning a separate git branch -D process per branch with evaluating the branches into an array and passing it directly to a single git branch -D call.

🎯 Why:
Calling native executables inside a loop in PowerShell spawns a new process and incurs overhead for every element. Batching them avoids the N+1 process problem.

📊 Impact:
Reduces process spawning overhead from O(N) to O(1) during the branch cleanup phase, which can be significantly faster when cleaning up a large number of local PR branches.

🔬 Measurement:
Run the script locally in a repository with hundreds of inactive local feature branches and observe the execution time decrease during the cleanup phase.

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PR created automatically by Jules for task 10516137096443176664 started by @Cylae

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