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- validate.ts: reject '_' everywhere (no longer valid)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/normalize-bun-monorepo.yaml

@@ -21,9 +21,12 @@ graph:
       role: package-metadata
       prompt: Biome setup failed ({{{reason}}}), but continue. Standardize package metadata for repo at {{{repoPath}}}.
   $START:
-    _:
+    new:
       role: workspace
       prompt: Set up bun workspace structure for repo at {{{repoPath}}}.
+    resume:
+      role: workspace
+      prompt: Review the previous run output and continue setting up the bun workspace structure for repo at {{{repoPath}}}.
   release:
     done:
       role: testing