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/main.py

@@ -11,6 +11,8 @@ from src.api.dashboard import router as dashboard_router
 from src.api.git_repositories import router as git_repositories_router
 from src.api.projects import router as projects_router
 from src.api.ssh_keys import router as ssh_keys_router
+from src.api.tool_instances import router as tool_instances_router
+from src.api.tool_instances import sessions_router
 from src.api.tool_types import router as tool_types_router
 from src.api.user_config import router as user_config_router
 from src.api.users import router as users_router
@@ -164,4 +166,6 @@ app.include_router(ssh_keys_router)
 app.include_router(git_repositories_router)
 app.include_router(user_config_router)
 app.include_router(tool_types_router)
+app.include_router(tool_instances_router)
+app.include_router(sessions_router)
 app.mount("/uploads", StaticFiles(directory="uploads"), name="uploads")