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- validate.ts: reject '_' everywhere (no longer valid)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

examples/debate.yaml

@@ -53,7 +53,8 @@ roles:
       required: [$status, argument]
 graph:
   $START:
-    _: { role: "against", prompt: "Present your opening argument against the proposition." }
+    new: { role: "against", prompt: "Present your opening argument against the proposition." }
+    resume: { role: "against", prompt: "Review the previous debate output and continue the argument against the proposition." }
   against:
     conceded: { role: "$END", prompt: "The against side conceded. Debate over." }
     continue: { role: "for", prompt: "Counter the opposing argument: {{{argument}}}" }