convert→SendMessage 'no transcript to resume' misleads agents into a phantom-GC hunt

[coryking]

Problem

When convert_session (session_to_subagent) is followed by SendMessage(to: <created_id>) in a session whose agent-teams resume runtime isn't functional, the harness returns:

{"success":false,"message":"Agent \"…\" has no transcript to resume. It may have
 been cleaned up. (No transcript found for agent ID: …)"}

The file is written correctly and is still on disk — nothing was cleaned up. But the "may have been cleaned up" wording (harness-side) + the existence of our pristine-artifact reaper led a field agent to conclude a garbage collector was eating its artifacts. It re-converted three times, all "failed" the same way, then abandoned the resume path entirely and wasted many turns. Reproduced here in an agent-teams-enabled session: convert→SendMessage works first try (the artifact wakes and answers), confirming the field failure was the session's missing/partial agent-teams runtime, exactly as convert_session's docstring already warns.

What we could do on our side

We can't change the harness's error string, but we can keep agents from chasing the phantom:

• A lightweight preflight the skill/guidance recommends — convert a throwaway and SendMessage it ONCE to confirm the resume channel works before building a multi-variant flow on it.
• convert_session's response could surface a hint about whether SendMessage is likely to resolve (e.g. note that a "no transcript to resume" result means agent-teams isn't active, NOT that the artifact was deleted — the file is intact and delete_conversions/the reaper never touch in-use forks).
• Possibly a one-line note in the cc-explorer skill's conversion section distinguishing this from the reaper.

Breadcrumb

Field session bluetaka 22876bad; the convert→SendMessage detour preceded the rewind issues fixed in #44/#45.