ci: add Forgejo security workflow (mirror of pre-commit hook)

Runs `tools/security/check-staged.sh ci` on every push to main
and every PR. Same script as the local pre-commit hook so the gate
is identical whether or not a contributor activated the local
hook via `bash tools/install-hooks.sh`. No Flutter / Dart toolchain
needed in the runner — the check is plain bash + git.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

.forgejo/workflows/security.yml

@@ -0,0 +1,45 @@
+name: Security
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  security:
+    name: Security check
+    runs-on: ubuntu-latest
+    steps:
+      # Same external-URL checkout dance the fai/platform CI uses —
+      # the runner's internal forgejo URL is not resolvable from
+      # inside the DinD job container. See `reference_forgejo_ci_setup`
+      # in the operator memory for the rationale.
+      - name: Checkout via external URL
+        run: |
+          set -eu
+          mkdir -p "$GITHUB_WORKSPACE"
+          cd "$GITHUB_WORKSPACE"
+          git init -q
+          git remote add origin \
+            "https://x-access-token:${GITHUB_TOKEN}@git.flemming.ws/${GITHUB_REPOSITORY}.git"
+          # Fetch enough history that scanning back to the merge base
+          # / origin/main yields a non-empty diff. depth=50 covers any
+          # realistic PR; larger PRs still pass since the script
+          # falls back to the root commit on missing base.
+          git fetch --depth=50 origin "$GITHUB_SHA"
+          git checkout -q FETCH_HEAD
+
+      - name: Security check (secrets, confidentiality, banned phrases)
+        # Same script `tools/security/check-staged.sh` the local
+        # pre-commit hook runs — defense in depth, so a contributor
+        # without `bash tools/install-hooks.sh` still cannot land
+        # secrets or banned phrases on main.
+        run: |
+          set -eu
+          BASE="origin/main"
+          if [ -n "${GITHUB_BASE_REF:-}" ]; then
+            git fetch --depth=50 origin "$GITHUB_BASE_REF"
+            BASE="origin/$GITHUB_BASE_REF"
+          fi
+          bash tools/security/check-staged.sh ci "$BASE"