WIP: working-copies backend cleanup

- Remove clone_mode/branch from API responses and make DB columns nullable
- Remove legacy clone-mode branches from create_tool_instance
- Add WORKSPACE_PATH compose variable alongside REPO_PATH
- Add workspace migration helpers in WorkspaceManager

Remaining: POST /workspaces/:id/instances, frontend clone_mode cleanup, tests

apps/api/src/models/tool/tool_instance.py

@@ -53,9 +53,11 @@ class ToolInstance(UUIDPrimaryKeyMixin, TimestampMixin, Base):
     )
     image_tag: Mapped[str | None] = mapped_column(String(256), nullable=True)
     probe_result: Mapped[dict | None] = mapped_column(JSON, nullable=True)
-    clone_mode: Mapped[str] = mapped_column(String(20), nullable=False, default="mount")
+    clone_mode: Mapped[str | None] = mapped_column(
+        String(20), nullable=True, default=None
+    )
     branch: Mapped[str | None] = mapped_column(
-        String(255), nullable=True, default="main"
+        String(255), nullable=True, default=None
     )
     selected_config_profile_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(), ForeignKey("config_profiles.id", ondelete="SET NULL"), nullable=True