Copilot's subagent spawning (Task API) may not properly return completion
signals to the orchestrator, causing it to hang indefinitely waiting for
agents that have already finished their work.
Added <runtime_compatibility> section to execute-phase.md with:
- Runtime-specific subagent spawning guidance (Claude Code, Copilot, others)
- Fallback rule: if agent completes work but orchestrator doesn't get the
signal, treat as success based on spot-checks (SUMMARY.md exists, commits
present)
- Sequential inline execution fallback for runtimes without reliable Task API
Fixes#1128