ci: run workflows on merge_group events (#378)

## What changed

Adds `merge_group:` to the `on:` trigger list in the generated CI
workflow: `template/{% if git_platform=="github.com" %}.github{% endif
%}/workflows/ci.yml.jinja`. A single-line addition, nothing else — the
template repo's own `.github/workflows/ci.yml` (which tests the template
itself) is untouched, out of scope for this change.

## Why

No merge queue is enabled by default on projects generated from this
template, but if one were turned on, GitHub only runs a workflow against
`gh-readonly-queue/*` refs when that workflow explicitly subscribes to
the `merge_group` event. Neither `push` nor `pull_request` covers it, so
a queued PR would sit there indefinitely waiting on checks that never
start. This change makes generated projects' CI queue-ready ahead of
time.

## What this PR is *not*

This PR does **not** enable a merge queue for any project. Turning one
on is a repo-admin ruleset / branch-protection setting outside a
workflow file's control — that decision stays with whoever administers
each downstream repo. Until it's flipped on, this change is a safe
no-op: `merge_group` events simply never fire.

## Confirmed safe under `merge_group`

- The tag-gated pypi/release jobs stay off — under `merge_group` the ref
is `gh-readonly-queue/main/pr-N-<sha>`, which is `ref_type == 'branch'`,
not `tag`.
- The `main`-gated docs-deploy job stays off for the same reason
(`ref_name` under `merge_group` isn't `main`).

So no release or deploy can fire from a merge-queue run in any generated
project.

## Test suite

Ran `uv run --locked tox -e tests` before pushing. 4 pre-existing
failures remain (`docs`/Sphinx trying to fetch `switcher.json` from
`diamondlightsource.github.io`), all caused by this sandbox's outbound
network restrictions — reproducible on a clean checkout with no changes,
unrelated to this PR. `pre-commit` and `type-checking` pass. No test
snapshots or asserts on the generated CI workflow's content, so nothing
needed updating for this change.

## Downstream repos

This is the durable upstream fix. `bluesky/bluesky` and
`bluesky/ophyd-async` carry the identical `merge_group:` addition
directly (both currently generated from this template) until they next
re-run copier and pick it up from here:

- bluesky/bluesky#2055
- bluesky/ophyd-async#1411

---
🤖 Generated with [Claude Code](https://claude.com/claude-code)

---
_Generated by [Claude
Code](https://claude.ai/code/session_01Y9bkocE5cCMw4LC72sJFHm)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: coretl

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

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