feat: implement web terminal for tool instances

- Add TerminalSession backend service for docker exec subprocess management
- Add TerminalManager for WebSocket session lifecycle management
- Create WebSocket endpoint at /ws/tool-instances/{id}/terminal
- Add session cookie authentication and instance ownership verification
- Install xterm.js with fit and web-links addons
- Create TerminalComponent with xterm.js integration
- Create TerminalPage with full-screen terminal view
- Add terminal route at /instances/:id/terminal
- Add terminal button to InstanceList for running instances
- Add terminal and arrow-left icons to icon registry
- Add comprehensive terminal CSS styles (dark theme, responsive)

Quality gates: typecheck ✓, lint ✓, build ✓, Python syntax ✓

openspec/changes/tool-terminal/design.md

@@ -0,0 +1,139 @@
+# Tool Terminal - Design
+
+## Architecture
+
+```
+Browser                    Backend                    Container
+  │                          │                           │
+  │  WebSocket connect       │                           │
+  │─────────────────────────>│                           │
+  │                          │  docker exec -it bash     │
+  │                          │───────────────────────────>│
+  │                          │                           │
+  │  stdin (keystrokes)      │  stdin                    │
+  │─────────────────────────>│───────────────────────────>│
+  │                          │                           │
+  │  stdout/stderr           │  stdout/stderr            │
+  │<─────────────────────────│<───────────────────────────│
+  │                          │                           │
+  │  resize (cols, rows)     │  pty resize               │
+  │─────────────────────────>│───────────────────────────>│
+  │                          │                           │
+```
+
+## Component Design
+
+### Backend
+
+**TerminalManager:**
+- Manages active terminal sessions
+- Maps WebSocket connections to container processes
+- Handles session lifecycle (create, resize, cleanup)
+
+**WebSocket Endpoint:**
+- `GET /ws/tool-instances/{instance_id}/terminal`
+- Authenticates user via session cookie
+- Establishes bidirectional WebSocket
+- Spawns `docker exec -it` with pseudo-TTY
+
+**Docker PTY:**
+- Uses `docker exec` with TTY allocation
+- Streams stdin/stdout/stderr via subprocess
+- Handles resize via `stty` or docker API
+
+### Frontend
+
+**TerminalComponent:**
+- Wraps xterm.js terminal
+- Manages WebSocket connection
+- Handles terminal resize
+- Fits container to parent element
+
+**TerminalPage:**
+- Full-page terminal view
+- Shows instance name in header
+- Connection status indicator
+- Reconnect on disconnect
+
+## Data Flow
+
+1. User clicks "Terminal" on running instance
+2. Frontend opens WebSocket connection
+3. Backend verifies ownership and spawns shell
+4. Bidirectional streaming begins
+5. User types → WebSocket → docker exec stdin
+6. Container output → docker exec stdout → WebSocket → xterm.js
+7. Resize events forwarded to adjust PTY dimensions
+
+## Session Lifecycle
+
+```
+Connect
+  │
+  ▼
+Authenticate ──> Reject (403)
+  │
+  ▼
+Spawn Shell
+  │
+  ▼
+Stream I/O ◄───> Resize
+  │
+  ▼
+Disconnect
+  │
+  ▼
+Cleanup Process
+```
+
+## Access Control
+
+- WebSocket handshake validates session cookie
+- Backend verifies user owns the instance
+- Reject connection with 403 if unauthorized
+- Close connection if instance stops running
+
+## Technical Details
+
+**Backend Libraries:**
+- `asyncio` for WebSocket handling
+- `subprocess` with `docker exec -it`
+- `fcntl` for PTY resize (Linux)
+
+**Frontend Libraries:**
+- `xterm` - Terminal emulator
+- `xterm-addon-fit` - Auto-fit to container
+- `xterm-addon-web-links` - Clickable URLs
+
+**Docker Commands:**
+```bash
+# Spawn shell
+docker exec -it {container_id} /bin/bash
+
+# Alternative with explicit TTY
+docker exec -i {container_id} sh -c 'exec bash'
+```
+
+## Error Handling
+
+- Connection refused → Show error message
+- Container not running → Disable terminal button
+- Shell spawn failed → Show error and close
+- Network disconnect → Attempt reconnect
+
+## CSS Integration
+
+```css
+.terminal-container {
+  width: 100%;
+  height: 100%;
+  min-height: 400px;
+  background: #1e1e1e;
+  border-radius: 8px;
+  overflow: hidden;
+}
+
+.terminal-container .xterm {
+  padding: 8px;
+}
+```