Guard workflow results against Promise clone failures

[Michaelliv]

Summary

• wait for started workflow subagents to settle before returning from the runtime
• reject non-structured-cloneable workflow results with a targeted forgot-to-await error
• validate runtime phase titles and agent prompt/options before they enter progress snapshots

Fixes #12

Verification

• npm test
• reproduced return { leaked: agent(...) } locally; now reports a workflow error instead of crashing Pi