refactor!: rename RoleDefinition.meta → frontmatter

BREAKING CHANGE: All workflow YAML files must use 'frontmatter' instead of 'meta'.

- workflow-protocol: RoleDefinition.meta → frontmatter, schema updated
- cli-workflow: validate.ts, workflow.ts — resolveMetaRef → resolveFrontmatterRef
- workflow-agent-kit: run.ts — metaSchema → frontmatterSchema
- All YAML files updated (examples/, .workflows/)

Fixes #374

examples/analyze-topic.yaml

@@ -19,7 +19,7 @@ roles:
     output: |
       Provide your analysis as markdown under the frontmatter.
       The frontmatter must include your structured findings.
-    meta:
+    frontmatter:
       type: object
       properties:
         thesis: