⚡ Bolt: [performance improvement] Fix N+1 file I/O for loadConfig in WorktreesService

[Donach]

💡 What: Hoisted loadConfig() evaluation out of the array mapping loop in WorktreesService.create().
🎯 Why: When generating multiple worktrees at once (bulk creation), applyWorktreeCreateDefaults previously executed await loadConfig() for every single item. This involves synchronous or asynchronous fs.readFile accompanied by parsing with yaml.load. This caused unnecessary $O(N)$ disk I/O and synchronous parsing overhead.
📊 Impact: Reduces disk I/O and CPU overhead significantly during multi-worktree instantiation. Instead of reading the YAML config from disk $N$ times, it only reads it once per request, converting $O(N)$ to $O(1)$.
🔬 Measurement: Verify via code inspection and by testing bulk creation via POST /worktrees array payload while profiling filesystem operations (e.g. strace or tracing).

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PR created automatically by Jules for task 6688627760371622729 started by @Donach

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