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- validate.ts: reject '_' everywhere (no longer valid)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/e2e-walkthrough.yaml

@@ -264,7 +264,8 @@ roles:
 
 graph:
   $START:
-    _: { role: "bootstrap", prompt: "Set up the Docker container and verify uwf is runnable." }
+    new: { role: "bootstrap", prompt: "Set up the Docker container and verify uwf is runnable." }
+    resume: { role: "bootstrap", prompt: "Review the previous run output and continue the walkthrough." }
   bootstrap:
     pass: { role: "config-and-registry", prompt: "Container {{{containerName}}} is ready. Validate config and workflow registration." }
     fail: { role: "$END", prompt: "Bootstrap failed: {{{error}}}. No container was created." }

.workflows/solve-issue.yaml

@@ -227,7 +227,8 @@ roles:
           required: [$status, error]
 graph:
   $START:
-    _: { role: "planner", prompt: "Analyze the issue and produce an implementation plan." }
+    new: { role: "planner", prompt: "Analyze the issue and produce an implementation plan." }
+    resume: { role: "planner", prompt: "Review the previous run output and continue the work." }
   planner:
     insufficient_info: { role: "$SUSPEND", prompt: "信息不足，需要补充：{{{reason}}}" }
     ready: { role: "developer", prompt: "Implement the TDD test spec (CAS hash: {{{plan}}}) in repo {{{repoPath}}}. Repo remote: {{{repoRemote}}}." }