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/proposal.md

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+# Tool Terminal
+
+## Problem
+
+Tool instances (code-server, jupyter-notebook, etc.) run in Docker containers but users have no way to access a shell inside those containers. This limits debugging, running ad-hoc commands, and managing the container environment.
+
+## Solution
+
+Provide browser-based terminal access to running tool containers via WebSocket:
+
+1. **WebSocket terminal sessions** - Real-time bidirectional communication
+2. **Pseudo-TTY** - Full terminal emulation with proper shell behavior
+3. **xterm.js frontend** - Professional terminal UI in the browser
+4. **Session management** - Multiple independent terminals per instance
+5. **Access control** - Only instance owners can access terminals
+
+## Key Features
+
+### Terminal Access
+- Open terminal from any running tool instance
+- Full bash/zsh shell inside the container
+- Standard terminal features (colors, cursor, history, etc.)
+
+### Real-time I/O
+- Instant character-by-character streaming
+- Stdout/stderr combined output
+- Support for interactive programs (vim, nano, etc.)
+
+### Terminal Resize
+- Dynamic column/row adjustment
+- Window resize handled gracefully
+- Proper text wrapping and scrolling
+
+### Session Management
+- Multiple terminals per instance
+- Independent sessions with isolation
+- Cleanup on disconnect
+
+## Benefits
+
+- **Debug containers** - Inspect running processes, check logs
+- **Run commands** - Execute ad-hoc scripts or tools
+- **Manage environment** - Install packages, edit config files
+- **No SSH needed** - Browser-based access from anywhere
+
+## Success Criteria
+
+- [ ] Terminal opens for any running instance
+- [ ] Commands execute and display output in real-time
+- [ ] Terminal resizes with browser window
+- [ ] Multiple terminals work independently
+- [ ] Sessions clean up on disconnect
+- [ ] Unauthorized users cannot access terminals