fix(swarm): auto-submit headed-spawn startup prompt after connect

A visible (headed) swarm spawn stages its initial prompt into the
client's input box with submit_input_on_startup=true, not via
queued_messages. The only production caller of process_remote_followups
(the post-connect dispatcher in reconnect.rs) gated on
has_queued_followups(), which ignores the startup-submission case. As a
result a freshly spawned headed agent showed its prompt but never sent
it, requiring a manual DM/wake to start.

Add has_pending_startup_submission() and include it in the post-connect
dispatch gate so the staged prompt is submitted once history loads.

Addresses the 'spawned agents do nothing' reports in #267, #268, #76.

Tests: regression test for the dispatch + predicate edge cases.