Agent Sessions Layout

Update the Revision History table at the bottom of LAYOUT.md with a dated entry describing what changed.

3. Implementation Principles

When proposing or implementing changes, follow these rules from the spec:

1. Maintain fixed positions — Do not add settings-based position customization
2. Panel must span the right section width — The grid structure places the panel below Chat Bar and Auxiliary Bar only
3. Sidebar spans full height — Sidebar is in the main content branch, spanning from top to bottom
4. New parts go in the right section — Any new parts should be added to the horizontal branch alongside Chat Bar and Auxiliary Bar
5. Preserve no-op methods — Unsupported features (zen mode, centered layout, etc.) should remain as no-ops, not throw errors
6. Handle pane composite lifecycle — When hiding/showing parts, manage the associated pane composites
7. Use agent session parts — New part functionality goes in the agent session part classes (SidebarPart, AuxiliaryBarPart, PanelPart, ChatBarPart, ProjectBarPart), not the standard workbench parts
8. Use separate storage keys — Agent session parts use their own storage keys (prefixed with workbench.agentsession. or workbench.chatbar.) to avoid conflicts with regular workbench state
9. Use agent session menu IDs — Actions should use Menus.* menu IDs (from sessions/browser/menus.ts), not shared MenuId.* constants

4. Key Files

5. Testing Changes

After modifying layout code:

1. Verify the build compiles without errors via the VS Code - Build task
2. Ensure the grid structure matches the spec's visual representation
3. Confirm part visibility toggling works correctly (show/hide/maximize)
4. Test that editors open in the ModalEditorPart overlay and that it closes properly
5. Verify sidebar footer renders with account widget