ci: port pi-devbox CI hardening — bash-default footgun guard + base-latest digest promote

Two CI-only changes ported from pi-devbox (no runtime/image impact),
adapted to opencode-devbox's split-base 2-variant pipeline. Rides the
next release.

C — eliminate the sh-vs-bash footgun class:
- Add `defaults: run: shell: bash` workflow-wide to docker-publish-split.yml
  and validate.yml. Gitea's default step shell is sh/dash, so bash-only
  syntax in a step that omits `shell: bash` fails silently. All pre-existing
  steps are POSIX, so bash runs them unchanged (no behavioural change).
- New .gitea/workflows/lint.yml (push/PR/dispatch): a Gitea-accurate shell
  guard (scripts/check-workflow-shell.sh) + pinned actionlint + shellcheck.
  The guard closes the actionlint blind spot: actionlint models GitHub
  (default shell bash) so it does NOT flag bash syntax in a shell-less step.
  Guard scans ALL .gitea/workflows/*.yml (hence the validate.yml default too).
  Ported from pi-devbox 26384fe/d1db595.

B — promote-base-latest re-points base-latest by digest, not need_build:
  The gate keyed off need_build=='true', assuming need_build==false meant
  base-latest was current. A dry-run dispatch that pre-builds base-<hash>
  falsifies that, leaving base-latest one base behind. Gate now runs on every
  tag release / promote dispatch; the no-op optimization moved into the step
  as a crane digest compare (re-tags only when base-latest != released
  base-<hash>). Ported from pi-devbox b7197e8.

Validated locally: all 3 workflows YAML-parse; shell guard passes real
workflows and correctly fails a synthetic omit-shell+pipefail workflow;
actionlint (pinned 1.7.7) passes with explicit .gitea/workflows/*.yml glob.

.gitea/workflows/docker-publish-split.yml

@@ -34,6 +34,17 @@ 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. (Ported from pi-devbox, where this class
+# bit twice: ed49b8d resolve-versions, b7197e8/b33e9dc promote-base-latest,
+# run 418.) All existing dash steps use only POSIX syntax, so bash (a
+# superset) runs them unchanged. Enforced by lint.yml's shell guard.
+defaults:
+  run:
+    shell: bash
+
 env:
   BUILDKIT_PROGRESS: plain
   IMAGE: ${{ vars.DOCKERHUB_USERNAME }}/opencode-devbox
@@ -519,11 +530,16 @@ jobs:
       - base-decide
       - build-variant-base
       - build-variant-omos
-    # Skip on cache-hit base builds: when need_build=false, base-latest
-    # already points at the same digest as base-<hash>, so the retag is
-    # a tautology and any transient failure of it is purely cosmetic.
-    # Manual workflow_dispatch with promote_latest=true overrides this
-    # gate as an escape hatch (e.g., if base-latest got hand-deleted).
+    # Run on every tag release (and promote_latest=true dispatch). The gate
+    # deliberately does NOT key off need_build anymore: the no-op optimization
+    # for genuine cache-hit releases moved INTO the step as a crane digest
+    # compare (see below). Keying the gate on need_build was wrong because a
+    # prior dry-run dispatch (promote_latest=false) can pre-build+push
+    # base-<hash>, making need_build=false on the subsequent tag run even
+    # though base-latest is still stale — the old gate then skipped promotion
+    # and left base-latest pointing at the PREVIOUS base. (Ported from
+    # pi-devbox b7197e8, which hit exactly this on its v1.2.3 dry-run-first
+    # release, 2026-06-27.)
     #
     # `always()` wrapper + explicit base-variant success check protects
     # against the gitea-Actions default of "skipped need => skip dependent":
@@ -532,8 +548,7 @@ jobs:
     if: |
       always() &&
       needs.build-variant-base.result == 'success' &&
-      (inputs.promote_latest == 'true' ||
-       (github.ref_type == 'tag' && needs.base-decide.outputs.need_build == 'true'))
+      (inputs.promote_latest == 'true' || github.ref_type == 'tag')
     runs-on: ubuntu-latest
     container:
       image: catthehacker/ubuntu:act-latest
@@ -558,11 +573,31 @@ jobs:
           crane auth login docker.io \
             -u ${{ vars.DOCKERHUB_USERNAME }} \
             -p "${{ secrets.DOCKERHUB_TOKEN }}"
-      - name: Re-tag base-<hash> as base-latest
+      - name: Re-tag base-<hash> as base-latest (only if stale)
+        env:
+          BASE_HASH_REF: ${{ env.IMAGE }}:${{ needs.base-decide.outputs.base_tag }}
+          BASE_LATEST_REF: ${{ env.IMAGE }}:base-latest
         run: |
-          crane copy \
-            ${{ env.IMAGE }}:${{ needs.base-decide.outputs.base_tag }} \
-            ${{ env.IMAGE }}:base-latest
+          set -euo pipefail
+          # Correctness invariant: after a release, base-latest must resolve to
+          # the SAME digest as the base-<hash> the just-built variants were
+          # FROM. Compare digests rather than trusting need_build — a prior
+          # dry-run dispatch can pre-build base-<hash>, so need_build=false on
+          # the tag run does NOT imply base-latest is already current. When the
+          # digests already match (genuine cache-hit release) this is a no-op,
+          # so we skip the crane copy entirely — preserving the original
+          # "don't do a tautological retag" intent and avoiding any cosmetic
+          # transient-failure exposure on releases that change nothing.
+          want=$(crane digest "${BASE_HASH_REF}")
+          have=$(crane digest "${BASE_LATEST_REF}" 2>/dev/null || echo "")
+          echo "base-<hash> digest: ${want}"
+          echo "base-latest digest: ${have:-<absent>}"
+          if [ "${want}" = "${have}" ]; then
+            echo "base-latest already current; nothing to promote."
+          else
+            echo "Promoting base-latest -> ${BASE_HASH_REF}"
+            crane copy "${BASE_HASH_REF}" "${BASE_LATEST_REF}"
+          fi
 
   # ── Phase 6: update Hub description (only on real release runs) ────
   update-description: