Default manual image builds to dev-<commit> when no tag is given

A manual run with an empty tag input failed validation. It now uses the
checked-out commit's short SHA, and the workflow input is optional.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

scripts/image-publication.mjs

@@ -9,7 +9,7 @@ export function imagePublication({ event, ref, inputTag, revision }) {
 
   let tag, channel;
   if (event === "workflow_dispatch") {
-    tag = inputTag ?? "";
+    tag = inputTag || `dev-${revision.slice(0, 7)}`;
     const commit = /^dev-([0-9a-f]{7,40})$/.exec(tag)?.[1];
     if (!commit || tag !== tag.trim())
       throw new Error(