Prevent raw internal subagent events from leaking into resumed session context

[Hmbown]

Summary

Resuming a session can surface raw internal subagent lifecycle markup and a full child handoff in the resumed context, rather than a concise, trusted resume state.

Observed in v0.9.0

A resumed task transcript included a raw <codewhale:runtime_event kind="subagent_completion"> payload, its internal-control instructions, and the child completion summary. This makes the resume path confusing and risks treating control-plane material as ordinary user-facing context.

Expected

• Internal runtime events remain control-plane data and are never rendered or replayed as ordinary conversation context.
• Resume should show a concise, provenance-preserving summary of child state/results.
• A user-authored reference file (such as a takeover prompt) must remain reference material, not be confused with a runtime resume protocol.

Acceptance criteria

• Regression test resumes a task containing a completed child and verifies no raw runtime-event envelope or internal-only directions reach the model/UI transcript.
• Resume retains the actual user task, child result summary, and checkpoint without changing authority/provenance.
• UI distinguishes completed, failed, cancelled, and still-running children clearly.