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/hooks/mutations/useSpawnSession.ts

@@ -8,6 +8,8 @@ type SpawnInput = {
     directory: string
     agent?: 'claude' | 'codex' | 'gemini'
     yolo?: boolean
+    sessionType?: 'simple' | 'worktree'
+    worktreeName?: string
 }
 
 export function useSpawnSession(api: ApiClient | null): {
@@ -22,7 +24,14 @@ export function useSpawnSession(api: ApiClient | null): {
             if (!api) {
                 throw new Error('API unavailable')
             }
-            return await api.spawnSession(input.machineId, input.directory, input.agent, input.yolo)
+            return await api.spawnSession(
+                input.machineId,
+                input.directory,
+                input.agent,
+                input.yolo,
+                input.sessionType,
+                input.worktreeName
+            )
         },
         onSuccess: () => {
             void queryClient.invalidateQueries({ queryKey: queryKeys.sessions })