fix: remove _ single-exit for user roles

$START keeps _ (special entry node). All user-defined roles now require
explicit $status enum in frontmatter + matching graph keys.

- moderator: remove UNIT_STATUS fallback, error on missing $status
- validate: reject _ graph keys for non-$START roles
- validate-semantic: remove checkSingleExitRole(), require $status enum
- update all test fixtures to use explicit status values
- fix examples/analyze-topic.yaml

Fixes #86

examples/analyze-topic.yaml

@@ -23,7 +23,7 @@ roles:
       type: object
       properties:
         $status:
-          enum: ["_"]
+          enum: ["done"]
         thesis:
           type: string
         keyPoints:
@@ -37,4 +37,4 @@ graph:
   $START:
     _: { role: "analyst", prompt: "Analyze the topic in the task and produce a structured summary with key points." }
   analyst:
-    _: { role: "$END", prompt: "Analysis complete. Finish the workflow." }
+    done: { role: "$END", prompt: "Analysis complete. Finish the workflow." }