feat: replace $START _ status with new/resume semantics

BREAKING: All workflow YAML files must update $START._ to $START.new + $START.resume.
The resume edge prompt replaces the previously hardcoded resume message.

- evaluate.ts: remove START_ROLE/START_STATUS special case, use $status like all nodes
- thread.ts: resolveEvaluateArgs passes 'new', cmdThreadResume passes 'resume'
- validate.ts: reject '_' everywhere (no longer valid)
- validate-semantic.ts: require 'new' and 'resume' edges on $START
- All workflow YAMLs and test fixtures updated

Fixes #101

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

@@ -57,10 +57,14 @@ roles:
         $status: { type: string, enum: ["ready"] }
 graph:
   $START:
-    _:
+    new:
       role: planner
       prompt: "Plan the work"
       location: null
+    resume:
+      role: planner
+      prompt: "Resume the work"
+      location: null
   planner:
     ready:
       role: $END
@@ -113,10 +117,14 @@ roles:
         $status: { type: string, enum: ["ready"] }
 graph:
   $START:
-    _:
+    new:
       role: planner
       prompt: "Plan"
       location: null
+    resume:
+      role: planner
+      prompt: "Resume"
+      location: null
   planner:
     ready:
       role: $END
@@ -156,10 +164,14 @@ roles:
         $status: { type: string, enum: ["ready"] }
 graph:
   $START:
-    _:
+    new:
       role: planner
       prompt: "Plan"
       location: null
+    resume:
+      role: planner
+      prompt: "Resume"
+      location: null
   planner:
     ready:
       role: $END