CI: run workflows on merge_group events

Add merge_group to the generated CI workflow's on: triggers so
projects built from this template get merge-queue-ready CI. Without
this, a GitHub merge queue would stall forever waiting on checks that
never start on its gh-readonly-queue/* refs.

The 4 pre-existing test failures (docs / sphinx switcher.json fetch)
are a sandbox network restriction unrelated to this change; no
snapshot test compares generated CI workflow content.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y9bkocE5cCMw4LC72sJFHm

Author: claude

template/{% if git_platform=="github.com" %}.github{% endif %}/workflows/ci.yml.jinja

@@ -7,6 +7,7 @@ on:
     tags:
       - '*'
   pull_request:
+  merge_group:
 
 jobs: