#567 gcs-datastore: registerNamespace does not detect conflicts with existing registrations

Description

When two repos register the same namespace slug against the same GCS prefix, the @swamp/gcs-datastore extension's registerNamespace silently succeeds on the second registration instead of throwing a conflict error.

The @swamp/s3-datastore extension correctly detects the conflict — its registerNamespace checks for an existing .namespace.json manifest from a different repoId and throws. The GCS extension does not perform this check.

Steps to reproduce

1. Create two repos (repo A and repo B) with different repoIds
2. Both point at the same GCS bucket and prefix
3. Run swamp datastore setup extension @swamp/gcs-datastore --config '{"bucket":"...","prefix":"SAME"}' --namespace shared on repo A — succeeds, registers namespace
4. Run the same command on repo B — succeeds silently with "Registered namespace 'shared' in datastore" and no conflict warning

Expected behavior

Step 4 should throw an error matching S3 behavior: the namespace is already registered by a different repo.

Actual behavior

Step 4 succeeds. The second repo's manifest overwrites the first with no conflict detection.

Root cause

registerNamespace in the GCS extension writes the .namespace.json manifest without checking if one already exists from a different repoId. The S3 extension reads the existing manifest, compares repoIds, and throws on mismatch — the GCS extension should do the same.

Found during giga-swamp UAT testing (swamp-club#560).