First message: stop waiting for an exited launch, no retry without a requester (#1512 follow-ups) #1518

Follow-ups from the merge pass's reviews of #1512 (start an agent from Chat; omo round 2 unless noted).

• A first message whose launched agent exits before delivery waits instead of being held (CodeRabbit 4334853910, omo L3; server/core/pending.ts around 99 and 442): launchBound is never cleared, so bindLaunched answers "wait" forever; Send now replies input_not_ready and a queued copy waits silently (Discard still works). The author has a test-first fix.
• A first message whose page leaves during its own automatic send can be re-queued with no requester (omo L1; around 103, 131, 338-342): holdRequested cannot hold an in-flight item, and an agent_blocked / pending_wait result retries it into queued, where it heads the queue and stalls other devices' messages with no note. Exclude a first message with no present requester from that retry.
• Leaving the pane holds the first message in ordinary flows (omo L2, by design): switching tabs while the agent boots, or a phone putting the page away, holds it and asks for Send now. Consider letting the page's pane release wait on the bridge-owned first message.
• The learned process is foreground_processes[0] at the first event naming the kind, with no shell check (omo L5, depends on herdr's ordering).
• The demo's detach does not hold the first message as the bridge now does (omo L7).
• The Start in-flight guard test exercises startNewSession, not the dialog (omo round 1 M3).
• Extra currentIdentity reads per drain while a pane holds a first message (omo L4).