Confirm installer client#303 pre-flight still works with a hidden `client list`

[saadqbal]

Follow-up to #139 / #136. client list is now Hidden: true in the CLI but still callable — the installer's one-client-per-machine pre-flight (tracebloc/client scripts/lib/provision.sh:_account_owns_namespace) shells out to tracebloc client list --plain and greps namespace=<ns>.

Verify against a #136 build that:

• tracebloc client list --plain still prints the namespace=… field (hidden ≠ disabled);
• the installer's #303 refusal (foreign client already installed) still fires;
• the provision.bats suite (tracebloc/client) passes unchanged.

If hiding ever breaks the parse, either keep list visible or give the installer a dedicated internal ownership probe.