chore(ci): only run PRD number check for PRs into main

The require-numbered-prds gate previously ran on every pull request.
Scope its trigger to PRs whose base branch is main, so numbering is
only enforced at the point of merging into main.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/prd-number-check.yml

@@ -3,6 +3,7 @@ name: prd-number-check
 on:
   pull_request:
     types: [opened, reopened, synchronize]
+    branches: [main]
 
 jobs:
   require-numbered-prds: