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/lib/shortcuts/agent.ts

@@ -4,6 +4,7 @@ export function registerAgentShortcuts(
   cycleAgent: () => void,
   cycleAgentReverse: () => void,
   focusModelSelector: () => void,
+  openAgentSelector: () => void,
 ) {
   const isMac = () => navigator.platform.toLowerCase().includes("mac")
 
@@ -41,4 +42,13 @@ export function registerAgentShortcuts(
     description: "focus model",
     context: "global",
   })
+
+  keyboardRegistry.register({
+    id: "open-agent-selector",
+    key: "A",
+    modifiers: { ctrl: !isMac(), meta: isMac(), shift: true },
+    handler: openAgentSelector,
+    description: "open agent",
+    context: "global",
+  })
 }