feat: uwf cas put-text for storing plain text in CAS

- Register built-in text schema ({type: 'string'}) alongside workflow schemas
- Add cmdCasPutText command: uwf cas put-text <text>
- Update CLI reference in workflow-util
- Update solve-issue.yaml procedure to use put-text

Refs #380

.workflows/solve-issue.yaml

@@ -20,7 +20,7 @@ roles:
       2. Revise the test spec accordingly
 
       After producing the test spec:
-      1. Store it via `uwf cas put "<markdown content>"` and capture the returned hash
+      1. Store it via `uwf cas put-text "<markdown content>"` and capture the returned hash
       2. Put the hash in meta.plan (required when status=ready)
     output: "Output a brief summary of the test spec. Frontmatter must include: status (ready or insufficient_info) and plan (CAS hash of the test spec, required when status=ready)."
     frontmatter: