feat: add dev-dashboard — browser-based workmux frontend

Web dashboard (Bun + xterm.js) that wraps workmux CLI commands and
renders tmux windows in embedded browser terminals. Replaces direct
tmux navigation with a sidebar-based UI at localhost:5111.

- Bun HTTP server with REST API for worktree CRUD (add/rm/open/close/send)
- Bun.Terminal PTY API to attach to tmux grouped sessions per worktree
- xterm.js frontend with WebSocket bridge for real-time terminal I/O
- Scrollback buffer for reconnection, ResizeObserver for dynamic fitting
- Add direnv allow to worktree-env post-create hook for nix devshell

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/worktree-env

@@ -50,6 +50,12 @@ EOF
 
 echo "Created .env.local with ports: backend=$backend_port, frontend=$frontend_port"
 
+# --- Allow direnv so the nix devshell activates in pane commands ---
+if command -v direnv &>/dev/null && [ -f .envrc ]; then
+  direnv allow
+  echo "direnv allowed"
+fi
+
 # --- Create matching windmill-ee-private worktree ---
 # Check parent directory first (sibling to worktree root), then fall back to home
 parent_dir="$(cd "$(pwd)/.." && pwd)"