experimental.chat.system.transform fires for task-spawned subagent sessions
after the task prompt (user message) is already in the conversation. Pushing
to output.system at that point places a system message at a non-zero position,
which Qwen3.6's strict Jinja chat template rejects with 'System message must
be at the beginning.'
Move session-start.sh injection to chat.message (first turn guard via
initializedSessions). Injected as a synthetic text part via unshift(), which
has no position constraints.
Brydon DeWitt
f0d21e9895