Add responsive compact layout for agent sessions web view

[Copilot]

The agent sessions web entry point renders well on desktop but breaks on phones and narrow viewports. This adds a compact mode that activates below 600px width, producing a full-bleed single-column layout.

Changes

• Compact mode detection (workbench.ts): New COMPACT_WIDTH_THRESHOLD (600px) and LayoutClasses.COMPACT class. Detected early in initLayout() for correct initial grid state, and dynamically in layout() on viewport resize. Only fires on state transitions — users can still manually toggle sidebar/aux bar while in compact mode.
• Auto-hide sidebar & auxiliary bar on entering compact mode, giving chat the full viewport width.
• Full-bleed CSS (style.css): In .compact, card margins, border-radius, and side borders are removed from chatbar, auxiliary bar, and panel. Chat content padding tightened (12→8px). Part appear transitions disabled.
• Titlebar adjustments (titlebarpart.css): Reduced toolbar padding in compact mode so the session picker has more room.

How to test

# Start the sessions web server
./scripts/code-sessions-web.sh --skip-welcome

# In the browser, resize to < 600px width (or use DevTools device emulation)
# Sidebar should auto-hide, cards should go full-bleed with no margins/radius
# Resize back to > 600px — card appearance restores, sidebar can be toggled back

[osortega]

@copilot are these changes scoped to only when running in web AND responsiveness only for small screens like mobile.
In other words, running in web on a machine that has a big enough viewport should still look the same as it does today

[Copilot]

@copilot are these changes scoped to only when running in web AND responsiveness only for small screens like mobile.
In other words, running in web on a machine that has a big enough viewport should s...

Yes, a big viewport looks identical to today. Every CSS rule is gated behind .agent-sessions-workbench.compact, and the .compact class is only added when the viewport width drops below 600px. On any viewport ≥ 600px, no compact rules apply and no layout changes occur.

The JS logic also only fires on state transitions — it doesn't run on every resize, only when crossing the 600px threshold. When entering compact mode it auto-hides the sidebar and auxiliary bar; when leaving compact mode those parts remain in whatever state the user left them (they don't force-show back).

Note: the changes apply to the sessions workbench on both web and desktop, which is intentional — if someone resizes the desktop sessions window very narrow, the layout should still adapt gracefully. But on a typical desktop viewport this never triggers.

[osortega]

@copilot hello