CI: single-trunk model — dev triggers on main, beta becomes RC-tag-triggered

Replaces the dev/beta branch split with one trunk (main): dev-track
builds still publish on every push, but the beta track now publishes
from a vX.Y.Z-rc.N prerelease tag instead of a separately-maintained
beta branch. Removes the branch nobody reliably kept in sync.

.forgejo/workflows/release.yml

@@ -11,6 +11,7 @@ env:
 
 jobs:
   build:
+    if: ${{ !contains(github.ref_name, '-rc.') }}
     runs-on: hestia
     defaults:
       run: