diff --git a/.gitea/workflows/ci.yml b/.gitea/workflows/ci.yml
index 358f3ab..a74ebec 100644
--- a/.gitea/workflows/ci.yml
+++ b/.gitea/workflows/ci.yml
@@ -1,6 +1,14 @@
 # CI on the self-hosted Gitea, ported from .gitlab-ci.yml during the move off
 # GitLab (2026-09-22). Gitea reads .gitea/workflows and ignores .github/ once
-# this directory exists; .github/workflows stays as it was for GitHub.
+# this directory exists.
+#
+# BUILD_ON: an org variable, set on both forges. Gitea push-mirrors this
+# repository to GitHub, and with BUILD_ON=github the shellcheck job here skips
+# and GitHub Actions runs the same lint on the mirrored copy
+# (.github/workflows/ci.yml). The `github` job at the end waits for the result
+# that workflow reports back, so this run still passes or fails with the lint.
+# Unset, everything runs here as before -- which is also the fallback if
+# GitHub is ever unavailable.
 #
 # Every job runs in an image pinned by digest (tag in the trailing comment),
 # and the only action used is coffey-labs/actions/checkout pinned by SHA. The
@@ -19,6 +27,7 @@ concurrency:
 
 jobs:
   shellcheck:
+    if: ${{ vars.BUILD_ON != 'github' }}
     runs-on: light
     # The -alpine variant, not koalaman/shellcheck:stable. That one is built
     # FROM scratch with shellcheck as the entrypoint and no shell at all, so a
@@ -34,3 +43,37 @@ jobs:
           [ -n "$files" ] || { echo "no shell scripts found"; exit 1; }
           echo "$files" | tr '\n' ' '
           shellcheck --severity=warning --format=gcc $files
+
+  # ------------------------------------------------------------ github ------
+  # With BUILD_ON=github the jobs above skip and GitHub Actions builds this
+  # commit on the push-mirrored copy (.github/workflows/ci.yml). This job waits
+  # for the commit status that workflow reports back, and passes or fails with
+  # it, so this run -- what pull requests and merges look at -- still carries
+  # the real result. If GitHub never reports, it times out and fails: the cue
+  # to unset BUILD_ON and build here again.
+  github:
+    if: ${{ vars.BUILD_ON == 'github' }}
+    runs-on: light
+    timeout-minutes: 150
+    container:
+      image: node:26-bookworm-slim@sha256:582460f614631b59b824ac6020533b9bf339c7fdf3a6d7db31abb6b4065f0212 # 26-bookworm-slim
+    env:
+      READ_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      SHA: ${{ github.event.pull_request.head.sha || github.sha }}
+      CONTEXT: ${{ github.ref_type == 'tag' && 'github/ci (tag)' || 'github/ci (branch)' }}
+    steps:
+      - run: apt-get update -qq && apt-get install -y -qq --no-install-recommends ca-certificates curl jq >/dev/null
+      - shell: bash
+        run: |
+          set -uo pipefail
+          url="${CI_SERVER_INTERNAL}/api/v1/repos/${GITHUB_REPOSITORY}/commits/${SHA}/statuses?limit=50"
+          echo "Waiting for '${CONTEXT}' on ${SHA}"
+          while :; do
+            # The newest status for the context is the one that counts.
+            s="$(curl -fsS -H "Authorization: token ${READ_TOKEN}" "$url" | jq -r --arg c "$CONTEXT" '[.[] | select(.context == $c)] | sort_by(.id) | last | "\(.status) \(.target_url)"' 2>/dev/null)"
+            case "$s" in
+              success\ *) echo "GitHub: ${s}"; exit 0 ;;
+              failure\ *|error\ *) echo "GitHub: ${s}" >&2; exit 1 ;;
+            esac
+            sleep 20
+          done
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..272411b
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,88 @@
+# ShellCheck on GitHub Actions, for the GitHub replica of this repository.
+# Gitea (git.coffeylabs.org) is the source of truth: it pushes every commit
+# here through a push mirror, and this workflow lints what arrives. Pull
+# requests live on Gitea, so there is no pull_request trigger -- a PR's branch
+# reaches this copy as an ordinary push.
+#
+# Every file in this repository is a shell script that people are invited to
+# run as root, so a lint gate is the cheapest guard against the class of bug
+# that keeps showing up here: unquoted expansions, values interpolated into
+# commands, and exit statuses that go unchecked.
+#
+# The switch is the org variable BUILD_ON, set on both forges. With
+# BUILD_ON=github every job here runs, the job in .gitea/workflows/ci.yml
+# skips, and Gitea's own run waits for the result this workflow reports back.
+# With it unset, every job here skips and Gitea lints as before -- which is
+# also the way back if GitHub is ever unavailable: unset it on Gitea.
+#
+# The job is .gitea/workflows/ci.yml's, in the same image pinned by digest,
+# with the same settings (severity=warning, gcc format, every script in one
+# run). The only action used is actions/checkout, pinned to a full commit SHA
+# with the release in a trailing comment.
+name: ci
+
+on:
+  push:
+    branches: ['**']
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  GITEA_URL: ${{ vars.GITEA_URL }}
+
+jobs:
+  # Tells Gitea a result is on its way, so the commit does not look unchecked
+  # while the checks run.
+  pending:
+    if: ${{ vars.BUILD_ON == 'github' }}
+    runs-on: ubuntu-latest
+    steps:
+      - env:
+          GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
+          CONTEXT: github/ci (${{ github.ref_type }})
+        run: |
+          jq -n --arg c "$CONTEXT" --arg u "$GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID" \
+            '{state:"pending", context:$c, target_url:$u, description:"GitHub Actions"}' |
+          curl -fsS -o /dev/null -X POST -H "Authorization: token $GITEA_TOKEN" -H 'Content-Type: application/json' \
+            --data @- "$GITEA_URL/api/v1/repos/$GITHUB_REPOSITORY/statuses/$GITHUB_SHA"
+
+  shellcheck:
+    if: ${{ vars.BUILD_ON == 'github' }}
+    runs-on: ubuntu-latest
+    # The -alpine variant, not koalaman/shellcheck:stable. That one is built
+    # FROM scratch with shellcheck as the entrypoint and no shell at all, so a
+    # job cannot run a script in it.
+    container:
+      image: koalaman/shellcheck-alpine:stable@sha256:c82fe42504fbc9fc68f15d36638e5ee2324ebb8b94e96a3c4e395bf361c49183 # stable
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      # Reproduces the settings ludeeus/action-shellcheck was given on GitHub:
+      # severity=warning, gcc format, every script in one run.
+      - run: |
+          files=$(find . -name '*.sh' -not -path './.git/*' | sort)
+          [ -n "$files" ] || { echo "no shell scripts found"; exit 1; }
+          echo "$files" | tr '\n' ' '
+          shellcheck --severity=warning --format=gcc $files
+
+  # One commit status on Gitea for the whole run. Gitea's ci.yml `github` job
+  # waits on exactly this context.
+  report:
+    if: ${{ always() && vars.BUILD_ON == 'github' }}
+    needs: [pending, shellcheck]
+    runs-on: ubuntu-latest
+    steps:
+      - env:
+          GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
+          CONTEXT: github/ci (${{ github.ref_type }})
+          STATE: ${{ (contains(needs.*.result, 'failure') || contains(needs.*.result, 'cancelled')) && 'failure' || 'success' }}
+        run: |
+          jq -n --arg s "$STATE" --arg c "$CONTEXT" --arg u "$GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID" \
+            '{state:$s, context:$c, target_url:$u, description:"GitHub Actions"}' |
+          curl -fsS -o /dev/null -X POST -H "Authorization: token $GITEA_TOKEN" -H 'Content-Type: application/json' \
+            --data @- "$GITEA_URL/api/v1/repos/$GITHUB_REPOSITORY/statuses/$GITHUB_SHA"
diff --git a/.github/workflows/shellcheck.yml b/.github/workflows/shellcheck.yml
deleted file mode 100644
index 855c32b..0000000
--- a/.github/workflows/shellcheck.yml
+++ /dev/null
@@ -1,34 +0,0 @@
-name: ShellCheck
-
-# Every file in this repository is a shell script that people are
-# invited to run as root, so a lint gate is the cheapest guard against
-# the class of bug that keeps showing up here: unquoted expansions,
-# values interpolated into commands, and exit statuses that go unchecked.
-on:
-  push:
-    branches: [main]
-  pull_request:
-  workflow_dispatch:
-
-permissions:
-  contents: read
-
-jobs:
-  shellcheck:
-    name: ShellCheck
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Run ShellCheck
-        uses: ludeeus/action-shellcheck@master
-        with:
-          # warning, not error: the first run at this level surfaced
-          # exactly two findings and both were fixed, so there is no
-          # pre-existing backlog to grandfather in. Anything new that
-          # trips it is genuinely new. Dropping to the default (info)
-          # would also pull in style suggestions across every script --
-          # worth doing, but as its own pass.
-          severity: warning
-          check_together: 'yes'
-          format: gcc