Required status checks name each matrix leg, so adding a Julia version silently leaves it ungated

[sotashimozono]

Branch protection on main requires six contexts, and four of them name a single matrix leg:

version / check-version
format / format-check
julia 1.11 — ubuntu-latest
julia 1.12 — ubuntu-latest
julia 1.12 — windows-latest
Docs build

The trap

Adding a Julia version to the matrix in .github/workflows/CI.yml does not add it to the
gate. The new leg runs, can fail, and the PR stays mergeable — silently, because nothing
connects the workflow to the protection settings. Renaming a leg is worse: the required context
then never reports, and main becomes permanently unmergeable until someone edits the
protection by hand.

TestShards.jl avoids this by requiring one aggregate context, test / All shards passed, and
letting the matrix change underneath it.

Options

1. Aggregate gate job. Add a job that needs the legs that must pass and require only that
   context. The matrix then changes freely. Costs one extra job per run, and the job has to
   needs a subset of the matrix — see below — so the matrix likely has to split in two.
2. Leave it, and write the coupling down. A comment in CI.yml saying that changing a job
   name or adding a version means editing branch protection. Cheaper, and honest, but it is a
   comment guarding a setting in a different system.

Deliberately excluded, and why

Not currently required, and this should stay true under either option:

• julia nightly — ubuntu-latest — continue-on-error, allowed to fail.
• julia 1.12 — macos-latest — sat in the free-tier queue for roughly 50 minutes on
  2026-09-01. Requiring it would block every merge for that long. It still runs, and it has
  passed on every run so far.

Both exclusions mean a regression on nightly or macOS can merge. That is the accepted cost; it
is recorded here so it is a decision rather than an oversight.