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/check-workflows.py

@@ -0,0 +1,110 @@
+#!/usr/bin/env python3
+"""Parse every workflow file and check the rules that only bite on Gitea.
+
+Two things this catches, both of which are silent where they happen:
+
+1. A workflow that does not parse produces NO RUN on Gitea — not a failed run,
+   not a cancelled one. It looks exactly like a runner that has not picked the
+   job up, so you wait, and waiting never fixes it.
+
+2. A job with a multi-cell matrix that is not gated to GitHub asks a single
+   self-hosted runner to do all of it at once. A sibling plugin measured six
+   jobs dying together with exit code 137 — SIGKILL, out of memory, two
+   seconds in, with not one error message in the log.
+
+It reports how many files and jobs it examined, and it is fatal when that is
+zero: "all workflows are fine" is also what this prints having parsed nothing,
+which is the failure mode the whole ci/ kit has now been bitten by four times.
+"""
+import sys
+import yaml
+
+# 🔴 Exact conditions, not a substring. The first version of this check asked
+# whether the `if:` CONTAINED the server_url test, and passed the very
+# condition it was written to catch:
+#
+#   github.event_name != 'schedule' || github.server_url == '...'
+#
+# — which gates the schedule and lets `pull_request` through, so five cells
+# still land on the Gitea runner on every PR that touches ci/. A substring
+# match is "adjacent to the signal" rather than the signal, which is the
+# failure mode this whole ci/ kit keeps being bitten by.
+ACCEPTED = {
+    # Gitea runs it only when a human clicks it.
+    "github.event_name == 'workflow_dispatch' || "
+    "github.server_url == 'https://github.com'",
+    # Or never on Gitea at all.
+    "github.server_url == 'https://github.com'",
+}
+
+def cells(job):
+    """Number of matrix cells, 1 when the job has no matrix."""
+    matrix = (job.get('strategy') or {}).get('matrix')
+    if not isinstance(matrix, dict):
+        return 1
+    if 'include' in matrix and isinstance(matrix['include'], list):
+        return max(len(matrix['include']), 1)
+    total = 1
+    for key, values in matrix.items():
+        if key in ('include', 'exclude') or not isinstance(values, list):
+            continue
+        total *= max(len(values), 1)
+    return total
+
+def main(paths):
+    problems = 0
+    files = 0
+    jobs = 0
+
+    for path in paths:
+        try:
+            with open(path) as handle:
+                doc = yaml.safe_load(handle)
+        except yaml.YAMLError as err:
+            print(f"::error file={path}::Does not parse, so on Gitea it will "
+                  f"simply never run: {err}")
+            problems += 1
+            continue
+
+        files += 1
+        if not isinstance(doc, dict):
+            continue
+
+        # Only .gitea/ is checked for the gate: .github/ workflows run on
+        # GitHub's runners, which are not the scarce resource here.
+        if not path.startswith('.gitea/'):
+            for _ in (doc.get('jobs') or {}):
+                jobs += 1
+            continue
+
+        for name, job in (doc.get('jobs') or {}).items():
+            jobs += 1
+            if not isinstance(job, dict):
+                continue
+            n = cells(job)
+            if n <= 1:
+                continue
+            condition = ' '.join(str(job.get('if', '')).split())
+            if condition not in ACCEPTED:
+                print(f"::error file={path}::Job '{name}' runs {n} matrix "
+                      f"cells on Gitea, and its `if:` is not one of the "
+                      f"accepted gates. One self-hosted runner is asked to do "
+                      f"all of it at once, and the symptom is exit code 137 "
+                      f"with an empty log. Found: {condition or '<none>'}")
+                problems += 1
+
+    # Not caller-configurable: the smallest repo that has workflows has one.
+    if files == 0:
+        print("No workflow file was parsed at all. The paths are wrong, so "
+              "this check proved nothing.", file=sys.stderr)
+        return 1
+
+    if problems:
+        print(f"\n{problems} workflow problem(s) across {files} file(s).")
+        return 1
+
+    print(f"  {files} workflow files parsed, {jobs} jobs checked")
+    return 0
+
+if __name__ == '__main__':
+    sys.exit(main(sys.argv[1:]))