ci: run workflows on merge_group events

[coretl]

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:

• CI: run workflows on merge_group events bluesky/bluesky#2055
• CI: run workflows on merge_group events bluesky/ophyd-async#1411

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🤖 Generated with Claude Code

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Generated by Claude Code

[coretl]

@ZohebShaikh is this the only change that blueapi had to make to use merge queues?

[ZohebShaikh]

Yes, that's it. This was only needed for our PR Conventional Commit Validation, which isn't in the copier template by default.