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 ✓

apps/api/src/models/__init__.py

@@ -2,8 +2,9 @@ from src.models.base import Base
 from src.models.git_repository import GitRepository
 from src.models.project import Project
 from src.models.ssh_key import SSHKey
+from src.models.tool_instance import ToolInstance
 from src.models.tool_type import ToolType
 from src.models.user import User
 from src.models.user_config import UserConfig
 
-__all__ = ["Base", "GitRepository", "Project", "SSHKey", "ToolType", "User", "UserConfig"]
+__all__ = ["Base", "GitRepository", "Project", "SSHKey", "ToolInstance", "ToolType", "User", "UserConfig"]