feat: add explicit Nextcloud release apply workflow

[vitormattos]

Summary

Add the explicit apply stage for reusable Nextcloud app releases.

• materialized, opt-in release-nextcloud-app.yml workflow;
• manual dispatch only, with execution restricted to the default branch definition;
• protected release environment support;
• reruns the tested release plan against the requested stable branch;
• validates consumer-owned publication credentials before mutation;
• creates the GitHub Release explicitly, which triggers the existing App Store build/publish workflow;
• catalog the workflow and prepare v0.3.0.

Together with the v0.2.0 artifact validation and App Store publication verification, this closes the generic release lifecycle tracked by LibreCodeCoop/github-governance#5.