feat: add git worktree session support with improved UI

Implement comprehensive worktree session support allowing users to spawn sessions in temporary git worktrees. Includes backend worktree management, full-stack integration, and refined UI for session type selection.

Backend:
- Add worktree creation/removal utilities with branch management
- Track worktree metadata (basePath, branch, name, path) in session metadata
- Automatic cleanup of worktrees when sessions fail or exit
- Enhanced error handling with stderr tail logging

UI improvements:
- Redesign session type toggle with improved alignment and spacing
- Move worktree description inline with label for cleaner layout
- Add branch name input field that appears when worktree mode selected
- Auto-focus on worktree input when switching modes
- Reduce gap between radio options from gap-3 to gap-1.5
- Update descriptive text and placeholders for clarity

Integration:
- Thread worktree parameters through API client, RPC handlers, and daemon
- Add worktreeEnv utility to read worktree info from environment
- Update session spawning to support both simple and worktree modes

web/src/api/client.ts

@@ -231,11 +231,13 @@ export class ApiClient {
         machineId: string,
         directory: string,
         agent?: 'claude' | 'codex' | 'gemini',
-        yolo?: boolean
+        yolo?: boolean,
+        sessionType?: 'simple' | 'worktree',
+        worktreeName?: string
     ): Promise<SpawnResponse> {
         return await this.request<SpawnResponse>(`/api/machines/${encodeURIComponent(machineId)}/spawn`, {
             method: 'POST',
-            body: JSON.stringify({ directory, agent, yolo })
+            body: JSON.stringify({ directory, agent, yolo, sessionType, worktreeName })
         })
     }
 }