Parse the workflows on Gitea too, and keep the compat matrix off its runner

Both found by the subscriptions session in its copy of this kit, and both
reproduce here. They share the shape everything else in ci/ has had: the
failure is invisible on GitHub, which is where anyone would be looking.

1. ci/workflows-in-sync.sh never parsed the workflow files on Gitea, and
   printed "Workflows: in sync." anyway. It skipped with a note when PyYAML was
   missing — and PyYAML is missing in exactly one place. Measured here rather
   than taken on report: `import yaml` on gitea/runner-images:ubuntu-latest
   raises ModuleNotFoundError. So the check was switched off on the single
   forge where an unparseable workflow produces no run at all, no failed run
   and no error to click on, which looks exactly like a runner that has not
   picked the job up.

   ci_python_yaml in ci/lib.sh now falls back to a container the way ci_php
   already did — alpine + py3-yaml, no network install, one apk call. There is
   no skip path left: workflows that cannot be parsed are a failure, not a
   note. Both routes verified to give byte-identical output.

2. compat.yml ran its five-cell matrix on Gitea for every PR touching ci/ or
   tests/. The `schedule` was gated to GitHub for exactly this reason and
   `pull_request` was forgotten. The subscriptions repo measured six jobs dying
   together with exit code 137 — SIGKILL, runner out of memory, two seconds in,
   without one error message in the log. Latent here only because this repo
   merges through GitHub and never opens a Gitea PR; one Gitea PR touching ci/
   would have found it.

New ci/check-workflows.py enforces both, called through ci_python_yaml. It
parses every workflow and fails when a Gitea job with more than one matrix cell
is not gated, and it is fatal when it parsed no file at all.

🔴 Its first version asked whether the `if:` CONTAINED the server_url test, and
passed the very condition it exists to catch: `!= 'schedule' || server_url ==
...` contains the gate while letting pull_request through. Substring matching is
adjacent to the signal rather than the signal. It now compares the whole
normalised condition against an allowlist, and was checked both ways in one run
— red against the unfixed compat.yml naming the job and the cell count, green
after.

Three self-test assertions added, 29 total: an unparseable workflow fails, an
ungated multi-cell matrix fails, and parsing zero files fails.

Two notes from building it, both the same trap this kit keeps setting. The
container path could not find check-workflows.py because ci_in_container tars
`git ls-files` and the file was untracked. And the ungated-matrix fixture used
| as a sed delimiter while the gate itself contains ||, so sed silently changed
nothing; the assertion reported "could not stage the fixture" instead of
passing, which is the only reason it was noticed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ci/lib.sh

@@ -96,6 +96,27 @@ ci_php() {
   ci_in_container "php:8.4-cli" "exec php $(printf '%q ' "$@")"
 }
 
+# Run one of this repository's python scripts, with PyYAML available.
+#
+# Same shape and the same reason as ci_php above, and it is not hypothetical:
+# the Gitea runner image has NO PyYAML (measured — `import yaml` raises
+# ModuleNotFoundError there), while every laptop and GitHub's hosted runners
+# have it. So a check that merely skips when the import fails is a check that
+# is switched off on exactly one forge — and Gitea is the forge where a
+# workflow that does not parse produces no run and no error to click on.
+#
+# alpine + py3-yaml rather than a python image + pip: no network install, one
+# apk call, the same pinned answer every time.
+ci_python_yaml() {
+  if python3 -c 'import yaml' 2>/dev/null; then
+    python3 "$@"
+    return
+  fi
+
+  ci_in_container "alpine:3.20" \
+    "apk add --no-cache python3 py3-yaml >/dev/null && exec python3 $(printf '%q ' "$@")"
+}
+
 ci_step() { printf '\n\033[1m→ %s\033[0m\n' "$*"; }
 ci_fail() { printf '\033[31mFAIL\033[0m %s\n' "$*" >&2; }