Amend ADR-015 and revert ADR-009: atomic-only slug registration

Summary

Amend the slug registration ADRs for the settled activation UX (single screen, immutability disclosure next to the slug field, one Enable click; format validated inline, availability only at form submission). The previously planned lease/confirm split is overengineering for that design: provisioning stays atomic and its refusal answers availability. No pre-check endpoint: it would have no caller.

• ADR-015 (internal): replace the lease/confirm registration flow with local pre-validation + atomic provisioning; the form submission is the immutability acknowledgment. MR: internal-handbook!8882.
• ADR-009: revert the slug availability endpoint added by handbook!20714. MR: handbook!20717.

The ADR-022 update originally scoped here is already done: the org-keyed slug discovery endpoint is gone from the published ADR, superseded by resolution by UUID plus the Rails-persisted mapping (gitlab#603023).