feat(maestro): server-side prompt queue for structured sessions

Prompts sent while a turn is active are queued on the daemon instead of
in the browser's React state. The queue is persisted in the session
sidecar and drained as the next turn when the current one completes.

Fixes the bug where queued messages were delivered to the wrong session
when switching focus in the UI: the client-side queue captured the new
sessionId in its drain closure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/maestro-cli/src/sessions.rs

@@ -346,6 +346,7 @@ mod tests {
             last_focused_at: None,
             acp_session_id: None,
             auto_approve: false,
+            queued_prompts: Vec::new(),
         }
     }