Track agent child processes, reap on completion

spawn_agent returns Child handle + log_path. AgentCycleState stores
the Child, polls with try_wait() on each trigger to detect completion.
No more filesystem scanning to track agent lifecycle.

AgentSnapshot (Clone) sent to TUI for display. AgentInfo holds the
Child handle and stays in the state.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/ui_channel.rs

@@ -126,7 +126,7 @@ pub enum UiMessage {
     ContextInfoUpdate(ContextInfo),
 
     /// Agent cycle state update — refreshes the F2 agents screen.
-    AgentUpdate(Vec<crate::subconscious::hook::AgentInfo>),
+    AgentUpdate(Vec<crate::subconscious::hook::AgentSnapshot>),
 }
 
 /// Sender that fans out to both the TUI (mpsc) and observers (broadcast).