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- validate.ts: reject '_' everywhere (no longer valid)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/eval-simple.yaml

@@ -25,6 +25,7 @@ roles:
       required: [$status, summary]
 graph:
   $START:
-    _: { role: "fixer", prompt: "Fix the code issue described in the task prompt." }
+    new: { role: "fixer", prompt: "Fix the code issue described in the task prompt." }
+    resume: { role: "fixer", prompt: "Review the previous run output and continue fixing the code issue." }
   fixer:
     done: { role: "$END", prompt: "Fix complete." }