Fail incomplete agent sandbox runs

[chubes4]

Summary

• Treat nested agent sandbox results as runtime failures when the nested agents/chat payload is still processing, has pending tools, or exhausts max turns without a terminal answer.
• Preserve the existing agent_runtime_incomplete failure contract while expanding detection beyond agent_runtime.result to raw nested payloads captured in stdout/stderr.
• Add smoke coverage for the Homeboy-style pending-tools payload and max-turn exhaustion payloads.

Trackers

• Fixes Agent sandbox run should fail or timeout when nested agent remains processing with pending tools #534
• Parent tracker: agent-task: prove release-channel Lab Codebox fanout end to end Extra-Chill/homeboy#3378
• Evidence run: homeboy-3378-release-channel-fanout-overlay-20260603015451

Verification

• npm run build
• npm run agent-runtime-failure-smoke
• npm run wordpress-plugin-smoke

Known local verification blocker

• npm run check could not complete reliably in this local Studio/Kimaki environment because artifact-contract-smoke repeatedly collided with long-lived/orphaned local Playground/OpenCode listener processes on random localhost ports. The focused checks above pass on the rebased branch.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Drafted the code changes and smoke coverage; Chris remains responsible for review and final validation.