feat: implement tool instances backend and session navigation

Backend:
- Create ToolInstance model with status tracking
- Add Alembic migration for tool_instances table
- Create Docker service for compose template rendering and container execution
- Add CRUD API endpoints for tool instances
- Add lifecycle endpoints (start/stop/restart)
- Add user sessions endpoint for navigation
- Register routers in main.py

Frontend:
- Create SessionsProvider with React context
- Create sessions API client
- Update AppShell with sessions section in navigation
- Add session status indicators and polling
- Add CSS for session navigation

Quality gates: typecheck ✓, lint ✓, build ✓

openspec/changes/tool-instances/proposal.md

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+# Tool Instances with Sessions
+
+## Problem
+
+Users currently have no way to launch development tools (code-server, Jupyter, etc.) directly from their repositories. The tool-types system exists but cannot create running container instances. Additionally, there's no concept of a "session" - a running tool linked to a specific repo that appears in navigation for quick access.
+
+## Solution
+
+Implement a complete tool instance management system with sessions:
+
+1. **ToolInstance Model** - Links a ToolType to a GitRepository with status tracking
+2. **Session Concept** - A running ToolInstance that gets a top-level navigation entry
+3. **Docker Integration** - Render compose templates and execute docker compose commands
+4. **Lifecycle Management** - Start, stop, restart, and delete instances
+5. **Navigation Integration** - Active sessions appear in the app shell for quick access
+
+## Key Features
+
+### Tool Instance Creation
+- Select a tool type and repository
+- Generate unique instance name
+- Render Docker Compose template with variables
+- Execute `docker compose up -d`
+- Store container metadata
+
+### Session Management
+- Sessions are active/running instances
+- Each session gets a top-level nav entry with the tool icon
+- Session dropdown in app shell shows active sessions
+- Clicking a session opens the tool in a new tab/window
+
+### Lifecycle Operations
+- Start: `docker compose start`
+- Stop: `docker compose stop` 
+- Restart: `docker compose restart`
+- Delete: `docker compose down -v` + remove DB record
+
+### Status Monitoring
+- pending, building, running, stopped, error
+- Real-time status via Docker API
+- Last accessed timestamp
+
+## Success Criteria
+
+- [ ] Create tool instances from repository page
+- [ ] Sessions appear in top-level navigation
+- [ ] Start/stop/restart/delete instances
+- [ ] Status monitoring works
+- [ ] Docker Compose templates render correctly
+- [ ] Navigation updates when sessions change