fix: restrict community submission workflows to labeled event only (#2741)

Both add-community-preset and add-community-extension workflows previously
triggered on issues opened, edited, and labeled events. This caused them to
fire on every new issue and post noisy bot comments explaining the issue
wasn't a submission (see #2739).

Changes:
- Narrow trigger from [opened, edited, labeled] to [labeled] only
- Update prompt instructions to stop silently on non-matching issues
  instead of posting a comment