Implement VSCode-style command palette (Cmd+Shift+P) with comprehensive command organization:
- 19 commands organized into 5 categories: Instance, Session, Agent & Model, Input & Focus, System
- Category-based grouping with proper visual hierarchy
- Keyboard shortcuts displayed for all applicable commands
- Search/filter by command name, description, keywords, or category
Add keyboard shortcuts:
- Cmd+Shift+A: Open agent selector
- Cmd+Shift+M: Open model selector (existing)
- Instance/Session navigation shortcuts (Cmd+[/], Cmd+Shift+[/])
Fix keyboard navigation:
- Model selector now highlights options with arrow keys using data-[highlighted] attribute
- Agent selector properly opens via keyboard shortcut
- Global keyboard handler skips Combobox/Select components to allow native navigation
Improve discoverability:
- Prominent centered Command Palette hint in connection status bar
- Keyboard shortcut hints next to agent and model selectors
Complete task 020-command-palette
