ci: stop required gate from rerunning on arbitrary label changes

[ArchonVII]

Acceptance Criteria

• examples/repo-required-gate.yml no longer starts the required gate on arbitrary PR label changes.
• ci:full remains documented as a force-full classifier label, with clear guidance that an already-open PR needs a normal rerun/event to pick up the changed label.
• Tests assert the caller does not include broad labeled / unlabeled triggers.
• Workflow/YAML validation and unit tests pass.