ci: add workflow_dispatch trigger

Allows manual pipeline runs from the Gitea UI without needing a
dummy commit. Also fixes blank-commit trigger issue (paths-ignore
skips runs with no changed files).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/publish.yml

@@ -6,6 +6,7 @@ on:
       - "README.md"
       - ".gitignore"
       - ".claude/**"
+  workflow_dispatch:
 
 jobs:
   publish: