feat: centralize conventional commit validation

[vitormattos]

Summary

Continue #15 by centralizing block-unconventional-commits as a reusable workflow with a thin organization-template caller.

The caller is pinned to immutable commit e2878c8c2d12d0d323116dedd2bcff37706a7eba.

Upstream alignment

Compared with the current Nextcloud template:

• use public ubuntu-latest;
• update checkout to v7.0.1;
• update webiny/action-conventional-commits to v1.4.2;
• use github.token explicitly instead of a secret lookup;
• retain the same pull-request event types and check name.

After merge this can be validated directly in LibreCodeCoop/extract.