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

packages/cli/src/__tests__/thread-location.test.ts

@@ -54,7 +54,7 @@ roles:
       type: object
       required: ["$status"]
       properties:
-        $status: { type: string }
+        $status: { type: string, enum: ["ready"] }
 graph:
   $START:
     _:
@@ -62,7 +62,7 @@ graph:
       prompt: "Plan the work"
       location: null
   planner:
-    _:
+    ready:
       role: $END
       prompt: "Done"
       location: null
@@ -110,7 +110,7 @@ roles:
       type: object
       required: ["$status"]
       properties:
-        $status: { type: string }
+        $status: { type: string, enum: ["ready"] }
 graph:
   $START:
     _:
@@ -118,7 +118,7 @@ graph:
       prompt: "Plan"
       location: null
   planner:
-    _:
+    ready:
       role: $END
       prompt: "Done"
       location: null
@@ -153,7 +153,7 @@ roles:
       type: object
       required: ["$status"]
       properties:
-        $status: { type: string }
+        $status: { type: string, enum: ["ready"] }
 graph:
   $START:
     _:
@@ -161,7 +161,7 @@ graph:
       prompt: "Plan"
       location: null
   planner:
-    _:
+    ready:
       role: $END
       prompt: "Done"
       location: null