feat: workspace backend foundation (PR-1)

- Add workspaces table migration (2026_06_01_add_workspaces)
- Create Workspace model with repo_id, user_id, branch, path, status
- Add workspace_id nullable FK to ToolInstance
- Create GitService for clone/fetch/pull/branch_exists_remotely
- Create WorkspaceManager for create/delete/sync lifecycle
- Create workspace CRUD API with 409 handling for duplicates and instances
- Wire workspace routes into FastAPI app
- 17 tests passing (8 unit + 9 integration), 1 skipped

Quality gates: ruff clean

apps/api/src/main.py

@@ -24,6 +24,7 @@ from src.api.tool_types import router as tool_types_router
 from src.api.notifications import router as notifications_router
 from src.api.user_config import router as user_config_router
 from src.api.users import router as users_router
+from src.api.workspaces import router as workspaces_router
 from src.config import Settings
 from src.models.notification import Notification  # noqa: F401 – Alembic model discovery
 from src.models.terminal_session import TerminalSessionModel  # noqa: F401 – Alembic model discovery
@@ -159,4 +160,5 @@ app.include_router(instance_proxy_router)
 app.include_router(terminal_router)
 app.include_router(events_router)
 app.include_router(notifications_router)
+app.include_router(workspaces_router)
 app.mount("/uploads", StaticFiles(directory="uploads"), name="uploads")