Add command palette with 19 commands and improved keyboard navigation

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

src/index.css

@@ -60,10 +60,12 @@ body {
 
 .connection-status {
   display: flex;
-  justify-content: flex-end;
+  justify-content: center;
+  align-items: center;
   padding: 8px 16px;
   background-color: var(--secondary-bg);
   border-bottom: 1px solid var(--border-color);
+  gap: 16px;
 }
 
 .status-indicator {