Feature: Async skill execution with deferred resolution

Summary

Allow slash commands/skills to execute asynchronously in the background while the user continues typing their prompt, resolving (blocking if needed) when the user submits.

Use Case

When loading a screenshot into context with a custom /ss skill:

Current behavior:

1. Type /ss
2. Wait for skill to execute (find file, read image)
3. Type prompt
4. Submit

Desired behavior:

1. Type /ss — starts executing in background
2. Continue typing prompt immediately
3. Submit — blocks only if skill hasn't finished, otherwise proceeds

Why This Matters

Drag-and-drop currently feels faster than skills for file loading because it doesn't block input. The async model would make skills competitive with drag-and-drop for common workflows like pulling in screenshots, logs, or other context.

Suggested UX

• Visual indicator showing skill is loading (spinner in input area?)
• If user submits before skill completes, show brief "waiting for /ss..." message
• Skills that fail should surface error before prompt executes

Additional Context

This would be particularly valuable for I/O-bound skills (file reads, fetches) where the latency is noticeable but the user's typing time could overlap with execution.