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:

examples/solve-issue.yaml

@@ -9,7 +9,7 @@ roles:
       - planning
     procedure: "Analyze the issue and create a detailed, actionable implementation plan."
     output: "Output the plan summary and list of concrete steps."
-    meta:
+    frontmatter:
       type: object
       properties:
         plan:
@@ -28,7 +28,7 @@ roles:
       - testing
     procedure: "Implement the plan. Write code, tests, and ensure existing tests pass."
     output: "List all files changed and provide a summary of the implementation."
-    meta:
+    frontmatter:
       type: object
       properties:
         filesChanged:
@@ -46,7 +46,7 @@ roles:
       - static-analysis
     procedure: "Review the implementation against the plan. Check for bugs, edge cases, and style."
     output: "Approve or reject with detailed comments explaining your decision."
-    meta:
+    frontmatter:
       type: object
       properties:
         approved: