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/session.rs

@@ -40,6 +40,16 @@ impl HookSession {
         self.state_dir.join(format!("{}-{}", prefix, self.session_id))
     }
 
+    /// Construct directly from fields.
+    pub fn from_fields(session_id: String, transcript_path: String, hook_event: String) -> Self {
+        HookSession {
+            state_dir: Self::sessions_dir(),
+            session_id,
+            transcript_path,
+            hook_event,
+        }
+    }
+
     /// Load from a session ID string
     pub fn from_id(session_id: String) -> Option<Self> {
         if session_id.is_empty() { return None; }