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/components/SessionList.tsx

@@ -162,6 +162,9 @@ export function SessionList(props: {
                         <div className="flex flex-wrap items-center gap-x-3 gap-y-1 text-xs text-[var(--app-hint)]">
                             <span>❖ {getAgentLabel(s)}</span>
                             <span>model: {getModelLabel(s)}</span>
+                            {s.metadata?.worktree?.branch ? (
+                                <span>worktree: {s.metadata.worktree.branch}</span>
+                            ) : null}
                             {(() => {
                                 const lastSeen = getLastSeenLabel(s)
                                 if (!lastSeen) return null