feat: workspace grid view (Cockpit Phase 1) (#327)

[sauravpanda]

Summary

Phase 1 of the Cockpit UX redesign (#327). Replaces the dashboard/home view with a grid of worktree cards.

What's new

• WorkspaceGrid component — responsive card grid showing all worktrees across all projects, grouped by project
• Status indicators — green dot (active), amber pulse (needs attention), gray (idle)
• Sidebar auto-hides — when no worktree selected, sidebar is hidden so the grid is the main view
• Cmd+Escape to return — from a terminal, press Cmd+Escape to go back to the grid (uses Cmd modifier so vim/less still work)

What's not in this phase

• Live terminal previews on cards (Phase 2)
• Status grouping by state (Phase 2)
• Supervisor agent integration (Phase 3)
• Natural language command bar (Phase 4)

Part of #327

Test plan

• Open app → see grid of all worktrees grouped by project
• Click a worktree card → opens terminal view
• Press Cmd+Escape from terminal → returns to grid
• vim/less inside terminal still receives plain Escape
• Sidebar appears only when a worktree is selected

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