Post-async-session.create, `session.create` returns in ~1ms with partial
info and the real agent fires `session.info` ~1s later. Previously the
status bar went straight to 'ready' right after the instant RPC return,
which was misleading — `prompt.submit` would block server-side waiting
for the agent to finish building.
Now:
- `newSession`: status = 'starting agent…' when info has no `version`,
else 'ready' (covers the fast resume path too)
- `session.info` event: flips status to 'ready' only if it was
'starting agent…', preserving running/interrupted/error states