ci: run push workflows only on develop, main and release tags

Every commit on a branch with an open PR ran the whole pipeline twice, once
for the push and once for the pull_request event, in separate concurrency
groups. Filter the push trigger to develop, main and v* tags so other
branches run CI and publish their images only through their PR. Tags are
listed explicitly because a branch filter alone stops tag pushes.

Update the trigger, benchmark and image prose in the README and the Gitea
Actions guide to match.

Refs #309

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -2,6 +2,8 @@ name: CI
 
 on:
     push:
+        branches: [develop, main]
+        tags: ['v*']
     pull_request:
         types: [opened, reopened, synchronize, edited]
     workflow_dispatch: