ci: retire the standalone web image and build-web step (#1444)

The gateway image now carries the SPA bundle, so web.Dockerfile,
scripts/build-web.mjs (+ test), and the publish build-web step go away.
The two CI-structure tests drop build-web from their expected-effects
lists. .env.example replaces the Next block with WEB_DIST_DIR docs.

.woodpecker/publish.yml

@@ -510,47 +510,3 @@ steps:
       # ERR_PNPM_OUTDATED_LOCKFILE despite a clean restore. This edge is the
       # serialization invariant; add it to every new workspace consumer.
       - publish-next-npm
-
-  build-web:
-    image: gcr.io/kaniko-project/executor:debug
-    when: *image_build_when
-    environment:
-      REGISTRY_USER:
-        from_secret: REGISTRY_USERNAME
-      REGISTRY_PASS:
-        from_secret: REGISTRY_PASSWORD
-      CI_COMMIT_BRANCH: ${CI_COMMIT_BRANCH}
-      CI_COMMIT_TAG: ${CI_COMMIT_TAG}
-      CI_COMMIT_SHA: ${CI_COMMIT_SHA}
-    commands:
-      - mkdir -p /kaniko/.docker
-      - echo "{\"auths\":{\"git.mosaicstack.dev\":{\"username\":\"$REGISTRY_USER\",\"password\":\"$REGISTRY_PASS\"}}}" > /kaniko/.docker/config.json
-      - |
-        DESTINATIONS="--destination git.mosaicstack.dev/mosaicstack/stack/web:sha-${CI_COMMIT_SHA:0:7}"
-        if [ "$CI_COMMIT_BRANCH" = "next" ]; then
-          if [ -n "$CI_COMMIT_TAG" ]; then
-            echo "[publish] FATAL: next web publish must be sha-only; refusing tag '$CI_COMMIT_TAG'" >&2
-            exit 1
-          fi
-          echo "[publish] next web publish is sha-only"
-        elif [ "$CI_COMMIT_BRANCH" = "main" ]; then
-          DESTINATIONS="$DESTINATIONS --destination git.mosaicstack.dev/mosaicstack/stack/web:latest"
-        elif [ -z "$CI_COMMIT_TAG" ]; then
-          echo "[publish] FATAL: web image publish may only run for main, next, or tag events" >&2
-          exit 1
-        fi
-        if [ -n "$CI_COMMIT_TAG" ]; then
-          DESTINATIONS="$DESTINATIONS --destination git.mosaicstack.dev/mosaicstack/stack/web:$CI_COMMIT_TAG"
-        fi
-        /kaniko/executor --context . --dockerfile docker/web.Dockerfile $DESTINATIONS
-    depends_on:
-      - build
-      - verify
-      # #1411: publish-next-npm mutates workspace manifests in place during
-      # its transform window and restores them at step end. Any step that
-      # reads the pipeline workspace (kaniko COPY of manifests, later
-      # installs) must run AFTER publish-next-npm, never concurrently —
-      # pipeline 2648 raced a COPY inside the window and failed
-      # ERR_PNPM_OUTDATED_LOCKFILE despite a clean restore. This edge is the
-      # serialization invariant; add it to every new workspace consumer.
-      - publish-next-npm