fix(workspace): claim detached sessions to source project

[kitlangton]

Summary

• Fix workspace session detach so sync ownership is claimed by the source project instead of the ambient request project.
• Add a regression test that invokes sessionWarp({ workspaceID: null }) from a workspace instance context and verifies ownership returns to the original project.

What Was Broken

When detaching a workspace-owned session back to local, sessionWarp used Instance.project.id as the fallback owner. If the request ran inside a workspace instance context, that ambient project was the workspace project rather than the source project, so the session row detached but the sync sequence owner was wrong.

Verification

• bun test --timeout 5000 test/control-plane/workspace.test.ts -t "sessionWarp detaches"