Sign the automated version bump commit

main requires signed commits, so the pull request the bump job opens after a
release cannot be merged while the commit in it is unsigned. The key the server
signs merge commits with is not reachable from a runner, so the job signs with a
dedicated release-bot SSH key that the instance trusts through
TRUSTED_SSH_KEYS — no bot account, because an account key is only consulted
after the web Verify flow and that flow has no API.

Inert until the key is trusted and RELEASE_BOT_SIGNING_KEY is set, and loudly so:
the step checks the secret and ssh-keygen before it starts, runs the key through
ssh-keygen -y so a truncated or re-wrapped one is caught as itself rather than as
"gpg failed to sign the data", and the commit is re-read for a gpgsig header
before it is pushed.

Co-Authored-By: Claude Opus 5 <[email protected]>

.gitea/workflows/publish.yml

@@ -13,6 +13,7 @@ name: Publish
 #   vars.IMAGE_NAME      optional, defaults to this repository's owner/name
 #   vars.REGISTRY_USER   optional, defaults to the actor running the workflow
 #   secrets.REGISTRY_TOKEN  required to push
+#   secrets.RELEASE_BOT_SIGNING_KEY  required to sign the version bump commit
 #
 # Point REGISTRY at a host the runner reaches directly, without an intermediate
 # proxy that caps request bodies: a browser image has layers well over 100MB,
@@ -218,6 +219,56 @@ jobs:
           echo "changed=true" >> "$GITHUB_OUTPUT"
           echo "package.json ${current} -> ${NEXT}"
 
+      # main requires signed commits, and a pull request carrying an unsigned
+      # one cannot be merged. The key the server signs merge commits with lives
+      # on the server and no runner can reach it, so the bump commit is signed
+      # here with a dedicated key the instance trusts through
+      # `[repository.signing] TRUSTED_SSH_KEYS`. Setting that up is in
+      # CLAUDE.md; nothing about it is committed here.
+      - name: Configure signing as the release bot
+        if: steps.bump.outputs.changed == 'true'
+        env:
+          SIGNING_KEY: ${{ secrets.RELEASE_BOT_SIGNING_KEY }}
+        run: |
+          set -euo pipefail
+
+          if [ -z "${SIGNING_KEY}" ]; then
+            echo "RELEASE_BOT_SIGNING_KEY is not set; the bump commit would be unsigned and unmergeable." >&2
+            exit 1
+          fi
+          if ! command -v ssh-keygen >/dev/null; then
+            echo "ssh-keygen is missing from this image; git cannot make SSH signatures without it." >&2
+            exit 1
+          fi
+
+          # The secret holds an OpenSSH private key. git signs by shelling out
+          # to ssh-keygen, which wants the key on disk beside the `.pub` it is
+          # pointed at, readable only by us, and rejects it unless the trailing
+          # newline survived the round trip through the secret store.
+          keydir="${RUNNER_TEMP:-${TMPDIR:-/tmp}}/release-bot-signing"
+          install -m 700 -d "${keydir}"
+          printf '%s\n' "${SIGNING_KEY}" | tr -d '\r' > "${keydir}/key"
+          chmod 600 "${keydir}/key"
+
+          # Doubles as a format check: a truncated, re-wrapped or
+          # passphrase-protected key fails here rather than as "gpg failed to
+          # sign the data" three steps later.
+          if ! ssh-keygen -y -f "${keydir}/key" </dev/null > "${keydir}/key.pub"; then
+            echo "RELEASE_BOT_SIGNING_KEY is not a usable OpenSSH private key (passphrase-protected, truncated, or re-wrapped on paste)." >&2
+            exit 1
+          fi
+
+          # A name that is not a person, and an address no account backs: the
+          # signature verifies against the trusted key rather than against a
+          # user, so this is a label on the commit and not an identity.
+          git config user.name 'Release Bot'
+          git config user.email '[email protected]'
+          # `gpg.format` is the historical name; `ssh` is what switches git to
+          # signing with the key above rather than with a GPG key.
+          git config gpg.format ssh
+          git config user.signingkey "${keydir}/key.pub"
+          git config commit.gpgsign true
+
       # The bump arrives as a pull request rather than as a commit straight to
       # main. Pushing a branch asks nothing of the task token beyond ordinary
       # write access, so nothing here depends on being allowed past whatever
@@ -236,14 +287,18 @@ jobs:
 
           branch="release/bump-${NEXT}"
 
-          # A name that is not a person, and a reserved address that can never
-          # resolve to one. Nothing here names the instance it runs on.
-          git config user.name 'Release bot'
-          git config user.email '[email protected]'
-
           git checkout -b "${branch}"
           git add package.json package-lock.json
           git commit -m "Set the working version to ${NEXT}"
+
+          # An unsigned commit would go unnoticed until someone tried to merge
+          # the pull request, so it fails here instead, where the cause is in
+          # front of you.
+          if ! grep -q "^gpgsig" <<<"$(git cat-file commit HEAD)"; then
+            echo "The bump commit came out unsigned; refusing to push it." >&2
+            exit 1
+          fi
+
           git push origin "${branch}"
 
           # node rather than jq to build the request body: jq is not in this