[BUG] Synchronous Unix command loading blocks UI during completion

[jlguo]

Issue: Synchronous Unix command loading blocks UI during completion

Problem Description

When users trigger command completion (e.g., typing / or @), the system scans all PATH directories synchronously, causing UI freezing/hanging. This results in delayed or unresponsive command suggestions, especially on systems with many PATH entries or slow filesystems.

Current Behavior

• Command completion triggers synchronous scanning of all PATH directories using readdirSync and statSync
• UI becomes unresponsive during scanning
• Users must wait for full PATH scanning to complete before seeing command suggestions
• No immediate fallback commands are provided

Expected Behavior

• Command completion should be responsive and non-blocking
• Users should see immediate basic command suggestions while full list loads in background
• UI should remain responsive during command loading

Root Cause

The loadSystemCommands function in src/ui/hooks/useUnifiedCompletion.ts (lines 118-175) uses blocking synchronous file operations:

• readdirSync and statSync for scanning PATH directories
• No event loop yielding during batch processing
• No immediate fallback command provision

Impact

• User Experience: Poor with laggy/unresponsive command completion
• Performance: UI freezes during PATH scanning
• Responsiveness: Slower startup/initialization of completion features

Files Affected

• src/ui/hooks/useUnifiedCompletion.ts (lines 118-175 - loadSystemCommands function)

Reproduction Steps

1. Open the CLI application
2. Type / to trigger command completion
3. Observe UI freezing/hanging while system scans PATH directories
4. Notice delayed appearance of command suggestions

Environment

• OS: Any (more noticeable on systems with many PATH entries)
• Application: CLI with command completion feature
• Affected Version: Current main branch

Priority

Medium-High - Affects core user interaction with command completion

Suggested Solution

Convert synchronous file operations to asynchronous with:

1. Immediate provision of minimal fallback commands
2. Async fs/promises operations (readdir, stat)
3. Batch processing with event loop yielding
4. Better error handling for individual file/directory failures