CI: add a manual run to deploy from

Woodpecker only offers Deploy on a successful pipeline, and the tests can't
pass until the builder image exists, so nothing could start the first build.

.woodpecker/manual.yaml

@@ -0,0 +1,17 @@
+# A run that always succeeds, to click "Deploy" on. Woodpecker only offers Deploy
+# on a successful pipeline, and check.yaml can't succeed until the builder image
+# exists, so this is how to build it the first time. It's also how to publish
+# when the newest push failed its tests.
+#
+# Trigger: click "Run pipeline" in Woodpecker and pick the branch.
+
+when:
+  - event: manual
+
+skip_clone: true
+
+steps:
+  - name: ready
+    image: alpine:3.22
+    commands:
+      - echo "Click Deploy on this pipeline and set the deploy target."