feat: add editor area tab support for Pixel Agents

[zxcvbnmmohd]

Summary

Enable Pixel Agents to open as full-screen editor tabs (WebviewPanel) in addition to the existing sidebar panel view. Users can now open the pixel art office anywhere in the editor area, just like terminals.

Changes

• Multi-webview Architecture: Refactored PixelAgentsViewProvider to support multiple simultaneous webviews (sidebar + editor tabs)
• Shared Agent State: Single agents Map broadcasted across all connected views for real-time synchronization
• New Command: pixel-agents.openEditorTab - Open office in full-screen editor tab
• UI Enhancement: Added "⊡ Tab" button to BottomToolbar for quick access
• Message Broadcasting: All agent state updates sync across sidebar and all open editor tabs
• Terminal Event Listeners: Global event listeners ensure all views stay synchronized with terminal lifecycle events
• Backward Compatible: 100% compatible with existing sidebar workflow

Features

✅ Open multiple Pixel Agents editor tabs simultaneously
✅ Arrange tabs in split panes alongside code editor
✅ All tabs stay perfectly synced with live agent updates
✅ Independent camera position per tab (local state)
✅ Seamless sidebar + editor tab coexistence
✅ Proper cleanup when tabs are closed

Technical Details

The implementation:

• Maintains single provider instance managing all state
• Uses WebviewPanel API for editor tabs (vs WebviewView for sidebar)
• Broadcasts messages to all connected webviews
• Tracks active webview for focus-aware message routing
• Registers global terminal event listeners (once per session)
• Properly cleans up editor tabs on close

Testing

✅ TypeScript compilation passes
✅ Build succeeds (esbuild + Vite)
✅ No new linting errors
✅ Backward compatible (existing tests unchanged)