## Why

Tool instances currently have inconsistent port configuration. OpenCode lacks a default port and doesn't expose a web interface, while code-server and jupyter have hardcoded ports. We need a systematic way to define tool ports and ensure OpenCode works properly via the web terminal interface.

## What Changes

- **Tool Port Configuration**: Make `default_port` required for all tool types and validate it during tool type creation
- **OpenCode Web Terminal**: Configure OpenCode to run a web server (e.g., on port 3000) so it can be accessed via browser, not just through the raw WebSocket terminal
- **Tunnel Port Discovery**: Ensure cloudflared tunnels use the correct internal port from the tool type definition
- **Terminal-First Tools**: Add support for tools that primarily use the terminal interface but may also expose a web UI
- **Tool Validation**: Add validation to ensure tool compose templates expose the port defined in `default_port`

## Capabilities

### New Capabilities
- `tool-port-configuration`: Systematic port definition and validation for tool types
- `opencode-web-server`: Running OpenCode with a web interface accessible via browser

### Modified Capabilities
- `tool-types`: Adding port validation requirements and web interface support for terminal tools
- `tool-instances`: Tunnel creation must read port from tool type configuration
- `tool-terminal`: Terminal tools may optionally expose web endpoints

## Impact

- Backend: Tool type model, validation, seed data, tunnel creation logic
- Frontend: Instance list may show both Open (web) and Terminal buttons for tools with dual interfaces
- Docker: OpenCode compose template needs a web server command
- Infrastructure: Cloudflared tunnels must target the correct internal port
