fix(workflows): ensure workflow-builder injects $ARGUMENTS in generated YAMLs

Fixes #1535

The workflow-builder's generate-yaml node did not explicitly require
generated workflows to reference $ARGUMENTS (or $USER_MESSAGE). When
the AI generated single-node workflows that accept user input, it
described the input in prose but omitted the $ARGUMENTS substitution
variable. The harness captured the user's invocation message but never
injected it into the node's conversation.

Changes:
- Add rule 13 to generate-yaml prompt: every workflow that accepts user
  input MUST reference $ARGUMENTS in at least one node prompt
- Add validation warning in validate-yaml when neither $ARGUMENTS nor
  $USER_MESSAGE appears in the generated YAML
- Regenerate bundled defaults

Author: kagura-agent

.archon/workflows/defaults/archon-workflow-builder.yaml

@@ -189,6 +189,7 @@ nodes:
       10. Use `allowed_tools: []` on classification/analysis nodes that don't need tools
       11. Use `denied_tools: [Edit, Bash]` when a node should only use Write (not edit existing files)
       12. Prefer `model: haiku` for simple classification tasks to save cost
+      13. **CRITICAL**: Every generated workflow that accepts user input MUST reference `$ARGUMENTS` (or `$USER_MESSAGE`) in at least one node prompt. For single-node workflows, include it directly in the prompt (e.g., `$ARGUMENTS` on its own line under a `## Input` or `## Request` heading). Without this, the user's invocation message is captured by the harness but never injected into the node's conversation — the agent sees an empty input.
 
       ## Output
 
@@ -223,6 +224,10 @@ nodes:
         exit 1
       fi
 
+      if ! grep -q '\$ARGUMENTS\|\$USER_MESSAGE' "$FILE"; then
+        echo "WARNING: workflow does not reference \$ARGUMENTS or \$USER_MESSAGE — user input will not be injected into node prompts"
+      fi
+
       echo "VALID"
     depends_on: [generate-yaml]