Wire AgentCycleState through runner and TUI

Runner owns AgentCycleState, calls trigger() on each user message
instead of the old run_hook() JSON round-trip. Sends AgentUpdate
messages to TUI after each cycle.

TUI F2 screen reads agent state from messages instead of scanning
the filesystem on every frame. HookSession::from_fields() lets
poc-agent construct sessions without JSON serialization.

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

src/agent/ui_channel.rs

@@ -124,6 +124,9 @@ pub enum UiMessage {
 
     /// Context loading details — stored for the debug screen (Ctrl+D).
     ContextInfoUpdate(ContextInfo),
+
+    /// Agent cycle state update — refreshes the F2 agents screen.
+    AgentUpdate(Vec<crate::subconscious::hook::AgentInfo>),
 }
 
 /// Sender that fans out to both the TUI (mpsc) and observers (broadcast).