The matrix bot would never have worked: wrong permissions, wrong forge

Two bugs, both of which would have failed silently on the first scheduled run:

- permissions were  while the step pushes a branch and opens a PR.
  It would have died with a 403 and I would have had a bot that never worked.
- it opened the PR on GITHUB, which is a one-way MIRROR. A PR merged there would be
  clobbered by the next fleet-repos mirror push from Gitea. A bot opening PRs against
  a mirror is a bot doing nothing, slowly.

Now: contents+pull-requests write, and the PR is opened on Gitea (canonical) via its
API. One long-lived PR, force-pushed in place -- a daily PR is the same mistake as a
daily comment, wearing a hat.

.github/workflows/compat.yml

@@ -27,7 +27,10 @@ on:
       - ".github/workflows/compat.yml"
 
 permissions:
-  contents: read
+  # write, because the matrix refresh pushes a branch and opens a PR. It does NOT get
+  # to move `main` -- that is the whole reason it is a PR. See the refresh step below.
+  contents: write
+  pull-requests: write
   issues: write
 
 jobs:
@@ -130,19 +133,27 @@ jobs:
       - name: matrix
         run: cat /tmp/matrix.md
 
-      # Keep the README's table true — but as a PULL REQUEST, not a push to main.
+      # Keep the README's table true — as a PULL REQUEST, on the CANONICAL forge.
       #
-      # This used to `git push origin HEAD:main` from CI. Unattended writes to main are
-      # exactly what the release barrier exists to prevent; a bot that can move main can
-      # move it somewhere nobody looked. A PR is reviewable, and nothing lands by itself.
+      # Two things this gets right that the obvious version gets wrong:
+      #
+      #  1. It is a PR, not a push to main. This used to `git push origin HEAD:main`
+      #     from CI. An unattended write to main is exactly what the release barrier
+      #     exists to prevent — a bot that can move main can move it somewhere nobody
+      #     looked. Nothing lands by itself.
+      #
+      #  2. It runs on GITEA, not GitHub. GitHub is a one-way MIRROR: a PR merged there
+      #     would be silently clobbered by the next `fleet-repos mirror` push from Gitea.
+      #     A bot opening PRs against a mirror is a bot doing nothing, slowly.
       #
       # A stale support matrix is not a stale doc — it is a false promise to somebody
-      # deciding whether to trust this with their backups. So it still gets refreshed
-      # daily; it just asks first.
-      - name: refresh the README matrix (as a PR)
-        if: ${{ github.event_name == 'schedule' && contains(github.server_url, 'github.com') }}
+      # deciding whether to trust this with their backups. So it is refreshed daily; it
+      # just asks first.
+      - name: refresh the README matrix (PR on the canonical forge)
+        if: ${{ github.event_name == 'schedule' && !contains(github.server_url, 'github.com') }}
         env:
-          GH_TOKEN: ${{ github.token }}
+          TOKEN: ${{ secrets.GITEA_TOKEN || github.token }}
+          API: ${{ github.server_url }}/api/v1/repos/${{ github.repository }}
         run: |
           python3 - <<'PY'
           import json, sys
@@ -153,25 +164,52 @@ jobs:
           print("changed" if compat.update_readme(rows) else "unchanged")
           PY
 
-          git diff --quiet -- README.md && { echo "matrix unchanged"; exit 0; }
+          git diff --quiet -- README.md && { echo "matrix unchanged — nothing to propose"; exit 0; }
 
           git config user.name  "truecloud-patch bot"
           git config user.email "bot@onetick.ninja"
 
-          branch="bot/compat-matrix"
-          git checkout -B "$branch"
+          BRANCH=bot/compat-matrix
+          git checkout -B "$BRANCH"
           git add README.md
           git commit -m "docs: refresh the TrueNAS compatibility matrix"
-          git push -f origin "$branch"
+          git push -f origin "$BRANCH"
 
-          # One long-lived PR, updated in place — not a new one every morning.
-          if ! gh pr view "$branch" --json number >/dev/null 2>&1; then
-            gh pr create --head "$branch" --base main \
-              --title "docs: refresh the TrueNAS compatibility matrix" \
-              --body "The daily compatibility check found that the support matrix in the README no longer matches what iXsystems' middleware actually looks like.
+          # ONE long-lived PR, force-pushed in place — not a new one every morning.
+          # (A daily PR is the same mistake as a daily comment, wearing a hat.)
+          BRANCH="$BRANCH" python3 - <<'PY'
+          import json, os, urllib.error, urllib.request
 
-          This PR only touches the block between the \`COMPAT MATRIX\` markers. It is regenerated by \`tools/compat.py --matrix --update-readme\` and force-pushed, so it always reflects the latest run."
-          fi
+          api, token, branch = os.environ["API"], os.environ["TOKEN"], os.environ["BRANCH"]
+          h = {"Authorization": f"token {token}", "Content-Type": "application/json"}
+
+          def call(url, method="GET", data=None):
+              r = urllib.request.Request(
+                  url, method=method, headers=h,
+                  data=json.dumps(data).encode() if data else None)
+              with urllib.request.urlopen(r) as resp:  # noqa: S310
+                  return json.load(resp) if resp.length != 0 else {}
+
+          existing = [
+              p for p in call(f"{api}/pulls?state=open")
+              if p["head"]["ref"] == branch
+          ]
+          if existing:
+              print(f"PR #{existing[0]['number']} already open; the force-push updated it")
+          else:
+              pr = call(f"{api}/pulls", "POST", {
+                  "head": branch, "base": "main",
+                  "title": "docs: refresh the TrueNAS compatibility matrix",
+                  "body": (
+                      "The daily compatibility check found that the support matrix in "
+                      "the README no longer matches iXsystems' actual middleware.\n\n"
+                      "This only touches the block between the `COMPAT MATRIX` markers. "
+                      "It is regenerated by `tools/compat.py --matrix --update-readme` "
+                      "and force-pushed, so it always reflects the latest run."
+                  ),
+              })
+              print(f"opened PR #{pr['number']}")
+          PY
 
       # ONE bug report, kept in sync. It is edited in place when the findings change and
       # says NOTHING when they do not.