## Why

Currently, each tool instance (e.g., pi-agent, code-server) supports exactly one terminal session. Users who want to run multiple concurrent tasks (e.g., a long-running build in one pane, an editor in another, and a shell for quick commands) must open multiple tool instances or use tmux/screen inside a single session. This is inefficient and confusing.

Additionally, the web terminal lacks basic usability features found in modern terminal emulators: fullscreen mode, detachable panes, session tabs, and keyboard shortcuts for common actions.

## What Changes

- **Backend**: Allow multiple `TerminalSession` objects per `ToolInstance`, each with a unique `session_id`
- **Backend**: Update `TerminalManager` to track and route multiple sessions per instance
- **Backend**: Update terminal WebSocket protocol to include `session_id` in connection URL or message
- **Frontend**: Add session tabs/management UI (create new session, switch between sessions, close sessions)
- **Frontend**: Add fullscreen mode for the terminal
- **Frontend**: Add keyboard shortcuts for session management (Ctrl+Shift+N new session, etc.)
- **Frontend**: Session list panel showing active sessions per instance

## Capabilities

### New Capabilities
- `multi-session-terminal`: Multiple independent terminal sessions per tool instance
- `terminal-fullscreen`: Fullscreen terminal mode
- `terminal-session-management`: Create, switch, rename, and close terminal sessions

### Modified Capabilities
- `tool-terminal`: Extend WebSocket protocol and UI to support multiple sessions per instance
- `terminal-session-lifecycle`: Session creation, naming, and cleanup for multi-session model

## Impact

- Backend: `TerminalManager`, `TerminalSession`, `api/terminal.py`, database schema (session tracking)
- Frontend: `TerminalComponent`, `terminal.tsx`, new `TerminalSessionTabs`, `TerminalSessionManager`
- Protocol: WebSocket message format changes (add session_id field)
- Database: New or extended table to track terminal sessions per instance
