ci: eliminate the sh-vs-bash footgun class (defaults + lint guard)

Root cause of the recurring 'Illegal option -o pipefail' failures
(ed49b8d resolve-versions; b7197e8 promote-base-latest, run 418):
docker-publish.yml had no workflow-level default shell, so Gitea's
sh/dash default applied and every bash-syntax step had to individually
remember 'shell: bash'.

- docker-publish.yml: add 'defaults: run: shell: bash' — fixes the whole
  class; all pre-existing dash steps are POSIX so bash runs them unchanged.
- lint.yml: new workflow, runs on every push/PR (not just release tags):
    * scripts/check-workflow-shell.sh — Gitea-accurate guard: fails if any
      run: step doesn't resolve to bash. Catches the omit-shell+bash-syntax
      case that actionlint MISSES (actionlint models GitHub, where the
      default shell is bash, so a shell-less step is assumed bash).
    * actionlint + shellcheck — catches explicit 'shell: sh' + bash syntax
      (SC3040) and general workflow errors.
  Verified locally: guard + actionlint pass current workflows; guard fails
  a synthetic omit-shell+pipefail workflow; shellcheck clean.

.gitea/workflows/docker-publish.yml

@@ -38,6 +38,16 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: false
 
+# Gitea Actions' default step shell is `sh -e {0}` (dash), which rejects
+# bash-only syntax like `set -o pipefail`, `[[ ]]`, and arrays. Setting the
+# default to bash workflow-wide eliminates the whole class of "forgot
+# `shell: bash` on this step" bugs (hit twice: ed49b8d resolve-versions,
+# b7197e8/b33e9dc promote-base-latest). All existing dash steps use only
+# POSIX syntax, so bash (a superset) runs them unchanged.
+defaults:
+  run:
+    shell: bash
+
 env:
   BUILDKIT_PROGRESS: plain
   IMAGE: ${{ vars.DOCKERHUB_USERNAME }}/pi-devbox

.gitea/workflows/lint.yml

@@ -0,0 +1,64 @@
+name: Lint workflows
+
+# Durable guard against CI-workflow bugs — most importantly the recurring
+# "bash-only syntax under the default `sh`/dash shell" footgun that broke
+# resolve-versions (ed49b8d) and promote-base-latest (b7197e8 → run 418).
+# actionlint runs shellcheck against each `run:` step using its *effective*
+# shell, so `set -o pipefail` under dash is flagged as SC3040 before any
+# expensive build runs. This is cheap (~10s) and independent of the build
+# pipeline, so it fires on every push/PR — not just on release tags, which
+# is where the build workflow (docker-publish.yml) is otherwise only
+# triggered.
+on:
+  push:
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: lint-${{ github.ref }}
+  cancel-in-progress: true
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  actionlint:
+    runs-on: ubuntu-latest
+    container:
+      image: catthehacker/ubuntu:act-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install shellcheck
+        run: |
+          apt-get update
+          apt-get install -y --no-install-recommends shellcheck python3-yaml
+
+      - name: Gitea shell guard (catches the actionlint blind spot)
+        # actionlint models GitHub Actions, where the default run shell is
+        # bash, so it does NOT flag bash syntax in a step that merely OMITS
+        # `shell:` — which is exactly how ed49b8d and b7197e8 manifested on
+        # Gitea (default sh/dash). This guard enforces that every run: step
+        # resolves to bash under Gitea's real defaults. Run it BEFORE
+        # actionlint so the more precise diagnostic surfaces first.
+        run: bash scripts/check-workflow-shell.sh .gitea/workflows
+
+      - name: Install actionlint (pinned)
+        env:
+          ACTIONLINT_VERSION: 1.7.7
+        run: |
+          curl -fsSL \
+            "https://github.com/rhysd/actionlint/releases/download/v${ACTIONLINT_VERSION}/actionlint_${ACTIONLINT_VERSION}_linux_amd64.tar.gz" \
+            | tar -xz -C /usr/local/bin actionlint
+          actionlint --version
+
+      - name: Run actionlint
+        # SHELLCHECK_OPTS excludes pure-style codes (quoting/style opinions)
+        # so the guard stays focused on correctness bugs — crucially the
+        # SC3xxx "not POSIX / wrong shell" family that catches the pipefail
+        # footgun. Do NOT exclude SC3040 (set -o pipefail under sh) or any
+        # other SC3xxx code.
+        env:
+          SHELLCHECK_OPTS: "-e SC2086 -e SC2016 -e SC2129 -e SC2001 -e SC2312"
+        run: actionlint -color