Command Palette: Add registry package search

[sbaker]

Summary

Extend the Command Palette (Ctrl+Shift+P) to search PrompdHub registry packages alongside slash commands and app commands.

Current behavior

• Command palette only searches slash commands and app commands
• Users must use the WelcomeView search bar or /install to find registry packages

Proposed behavior

• Typing in the command palette also searches registry packages (debounced, async)
• Registry results shown in a separate section with package icon, version badge, and description
• Selecting a registry result opens it in the Package Explorer (same as WelcomeView search)
• Results should use cached registry data first (instant), with live search as fallback

Considerations

• Async API calls during keystroke — needs debounce and loading indicator
• New result type with different rendering (package icon, version, description)
• Different action on select (download + open in explorer vs execute command)
• Should not slow down command/slash command matching

References

• Command palette: frontend/src/modules/components/CommandPalette.tsx
• Registry search: frontend/src/modules/components/RegistrySearchBar.tsx
• Registry sync cache: frontend/src/modules/lib/intellisense/registrySync.ts