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- validate.ts: reject '_' everywhere (no longer valid)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

workflows/solve-issue.yaml

@@ -283,9 +283,12 @@ roles:
         - error
 graph:
   $START:
-    _:
+    new:
       role: planner
       prompt: Analyze the issue and produce an implementation plan.
+    resume:
+      role: planner
+      prompt: Review the previous run output and continue the work.
   planner:
     insufficient_info:
       role: $SUSPEND