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/models/tool_instance.py

@@ -14,6 +14,7 @@ if TYPE_CHECKING:
     from src.models.project import Project
     from src.models.tool_type import ToolType
     from src.models.user import User
+    from src.models.workspace import Workspace
 
 
 class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
@@ -60,8 +61,12 @@ class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
         UUID(), ForeignKey("config_profiles.id", ondelete="SET NULL"), nullable=True
     )
     ssh_key_ids: Mapped[list[str] | None] = mapped_column(JSON, nullable=True)
+    workspace_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(), ForeignKey("workspaces.id", ondelete="SET NULL"), nullable=True
+    )
 
     tool_type: Mapped["ToolType"] = relationship()
+    workspace: Mapped["Workspace | None"] = relationship()
     repository: Mapped["GitRepository"] = relationship()
     project: Mapped["Project"] = relationship()
     owner: Mapped["User"] = relationship()