ci: remove dead hosted serial-linux job

serial-linux (hosted ubuntu-latest) has been `if: false` since 2026-07-30 and
never runs. Remove the dead job block and retire the dangling references:

- TODO(hosted-serial-ci) narrowed to the single remaining disabled hosted
  serial job (serial-macos); the hosted linux definition is gone.
- The cache producer comment no longer claims serial-linux refreshes the
  hosted pnpm/Playwright caches; there is currently no active master producer
  for them, so restores are cold on a lockfile change.
- The self-hosted standby's frozen-archive comment no longer cross-references
  the deleted job.
- serial-macos gains its own intro comment since the shared 'hosted reference
  jobs below are disabled' lede was removed.

No runner allocation, required gate, or all-checks-passed.needs reference this
job; the aggregate is unchanged.

.github/workflows/ci.yml

@@ -42,8 +42,10 @@ env:
 
 jobs:
 
-  # TODO(hosted-serial-ci): Re-enable the three hosted serial reference jobs before release.
-  # The self-hosted standby remains active on every master push.
+  # TODO(hosted-serial-ci): Re-enable the remaining disabled hosted serial reference job
+  # (serial-macos) before release. The hosted serial-linux definition was removed as dead
+  # code (it was `if: false`); the self-hosted standby lane below remains active on every
+  # master push.
 
   # Three enterprise jobs isolate coverage, static analysis, and the
   # build-backed consumer tail. The consumer job owns the only Linux build so
@@ -220,8 +222,10 @@ jobs:
           restore-keys: |
             ${{ runner.os }}-node-${{ env.PRIMARY_NODE_VERSION }}-pnpm-
 
-      # Pull requests restore the cache normally produced by serial-linux on
-      # master; they do not pay compression and upload on the required path.
+      # Pull requests restore the pnpm store and Playwright caches without paying
+      # compression and upload on the required path. There is currently no active
+      # master producer for these hosted caches — the hosted serial-linux job that
+      # refreshed them was disabled, so restores below are cold on a lockfile change.
       - uses: actions/cache/restore@v4
         if: vars.DSH_CI_FAILOVER_LINUX != 'selfhosted' || github.event.pull_request.user.login == 'dependabot[bot]'
         with:
@@ -492,76 +496,6 @@ jobs:
         shell: pwsh
         run: pnpm run check:ci:windows-complete
 
-  # The hosted reference jobs below are temporarily disabled; the self-hosted
-  # standby remains active. Each enabled host executes the complete, unsharded
-  # primary Node aggregate with one gate worker, giving reviewers a simple
-  # cross-platform oracle for completeness and timing.
-  serial-linux:
-    if: false
-    name: serial / linux
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v6
-        with:
-          fetch-depth: 2
-
-      - uses: pnpm/action-setup@v4
-        with:
-          dest: ${{ runner.temp }}/setup-pnpm
-
-      - uses: actions/setup-node@v6
-        with:
-          node-version: ${{ env.PRIMARY_NODE_VERSION }}
-
-      - name: Configure pnpm store path
-        id: pnpm-store
-        run: |
-          store_root="$HOME/.local/share/pnpm/store"
-          echo "PNPM_CONFIG_STORE_DIR=$store_root" >> "$GITHUB_ENV"
-          store_path=$(PNPM_CONFIG_STORE_DIR="$store_root" pnpm store path --silent)
-          echo "path=$store_path" >> "$GITHUB_OUTPUT"
-
-      # Master refreshes the pnpm store cache that pull requests restore without saving.
-      # The store cache stays a hand-rolled actions/cache step rather than
-      # setup-node's `cache: pnpm`: the enterprise pull-request jobs above
-      # restore exactly this key and path, and setup-node's built-in cache
-      # uses its own key format — converting this producer would silently
-      # starve their documented restore-only optimization.
-      - uses: actions/cache@v4
-        with:
-          path: ${{ steps.pnpm-store.outputs.path }}
-          key: ${{ runner.os }}-node-${{ env.PRIMARY_NODE_VERSION }}-pnpm-${{ hashFiles('pnpm-lock.yaml') }}
-          restore-keys: |
-            ${{ runner.os }}-node-${{ env.PRIMARY_NODE_VERSION }}-pnpm-
-
-      # Master produces the hosted Chromium cache restored by pull requests.
-      - uses: actions/cache@v4
-        with:
-          path: ~/.cache/ms-playwright
-          key: ${{ runner.os }}-playwright-${{ hashFiles('pnpm-lock.yaml') }}
-          restore-keys: |
-            ${{ runner.os }}-playwright-
-
-      - name: Install (immutable)
-        run: pnpm install --frozen-lockfile
-
-      - name: Install Playwright Chromium and system dependencies
-        run: pnpm --filter @deepseek-ai/dsh-web-frontend exec playwright install --with-deps chromium
-
-      - name: Prepare bubblewrap (unrestrict userns)
-        run: bash scripts/prepare-ci-bubblewrap.sh
-
-      - name: Run complete unsharded primary Node CI serially
-        env:
-          DSH_ARCHIVE_BASE_REF: ${{ github.event.before }}
-          DSH_COVERAGE_MAX_WORKERS: '1'
-          DSH_E2E_MAX_WORKERS: '1'
-          DSH_GATE_CONCURRENCY: '1'
-          DSH_OXLINT_THREADS: '1'
-          DSH_PUBLINT_CONCURRENCY: '1'
-          DSH_SNAPSHOT_MAX_CONCURRENCY: '1'
-        run: pnpm run check:ci:linux-primary
-
   # Hot-standby drill for the in-house self-hosted pool: every master move
   # re-runs the complete unsharded aggregate on the persistent 64-core VM,
   # continuously proving that environment can take over a required lane if
@@ -580,8 +514,9 @@ jobs:
     name: serial / linux (self-hosted standby)
     runs-on: [self-hosted, linux, x64, vm-backup]
     steps:
-      # Full history + DSH_ARCHIVE_BASE_REF below: same frozen-archive
-      # comparison as serial-linux. Depth 2 would miss github.event.before
+      # Full history + DSH_ARCHIVE_BASE_REF below: the same frozen-archive
+      # comparison used by the hosted serial reference. Depth 2 would miss
+      # github.event.before
       # on multi-commit or force pushes; full fetch is cheap here because
       # checkout resolves against the VM's local mirror.
       - uses: actions/checkout@v6
@@ -621,6 +556,8 @@ jobs:
           DSH_SNAPSHOT_MAX_CONCURRENCY: '1'
         run: pnpm run check:ci:linux-primary
 
+  # The sole remaining disabled hosted serial reference job (the hosted linux
+  # definition was removed as dead code); see TODO(hosted-serial-ci) above.
   serial-macos:
     if: false
     name: serial / macos