ci: trigger on merge_group (bootstrap the merge queue)

[wbarnha]

Description

Merge this first, directly onto master, to un-stall the merge queue.

The merge queue is enabled on master, but master's CI workflow only triggers on push / pull_request / release — not merge_group. A queue validates each candidate as a merge_group event, so the required check job never runs for queued PRs and merges stall. (The earlier trigger PR, #726, was based on the stacked chain and merged into that branch, not master, so master still lacks it.)

This is the same 4-line on: addition, but based directly on master so it can land without the whole chain:

on:
  push: { branches: [master] }
  pull_request: { branches: [master] }
  merge_group:          # <-- added
  release: { types: [created] }

How to land it (chicken-and-egg)

Because "require merge queue" is on, the normal Merge button will try to queue this PR too — which would stall for the same reason. Break the loop one of two ways:

• Admin bypass: merge this PR directly (repo admins can bypass the queue), or
• Temporarily turn off "Require merge queue", merge this PR, turn it back on.

Once this is on master, the queue is fully functional and everything else (including the stacked chain #716→#725) can go through it normally.

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (0a92897) to head (87af207).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #727   +/-   ##
=======================================
  Coverage   94.15%   94.15%           
=======================================
  Files         104      104           
  Lines       11136    11136           
  Branches     1201     1201           
=======================================
  Hits        10485    10485           
  Misses        550      550           
  Partials      101      101           

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