perf: make unix command loading non-blocking in completion hook

[jlguo]

Summary

• Performance improvement: Convert synchronous Unix command loading to asynchronous
• User experience: Eliminates UI blocking during PATH directory scanning
• Responsiveness: Provides immediate fallback commands while full list loads

Changes

• Replace readdirSync/statSync with async fs/promises operations
• Use minimal fallback commands immediately to avoid UI blocking
• Process PATH directories in batches with event loop yielding
• Add robust error handling for individual file/directory failures
• Preserve fallback commands on load failure instead of resetting

Benefits

1. No more UI freezing when triggering command completion
2. Immediate command suggestions via fallback commands
3. Better error resilience - individual failures don't break entire process
4. Improved responsiveness on systems with many PATH entries or slow filesystems

Technical Details

• Before: Synchronous blocking operations that freeze UI during PATH scanning
• After: Asynchronous non-blocking operations with progressive loading
• Batch processing: 10 entries per batch with setImmediate yielding
• Error handling: Individual file/directory errors caught and ignored

Test Plan

• Trigger command completion (/ or @) - should see immediate suggestions
• Verify UI remains responsive during command loading
• Test on systems with many PATH directories
• Verify fallback commands work when full loading fails

Coworker: @claude
Issue: #164