fix(dag): make YAML authoring explicit

[LeXwDeX]

Summary

• publish exact start, extend, and replan YAML envelopes in the on-demand block guide
• list only author-written YAML fields and keep runtime-owned fields out of authoring prompts
• validate every copyable guide example through the real WorkflowAuthoring seam

Verification

• packages/core: command plugin tests (20/20) + typecheck
• packages/opencode: workflow authoring fixtures (16/16) + typecheck
• pre-commit lint/typecheck gate passed