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/subconscious/hook.rs

@@ -135,11 +135,11 @@ pub struct AgentCycleOutput {
 }
 
 /// Per-agent runtime state visible to the TUI.
+#[derive(Clone, Debug)]
 pub struct AgentInfo {
     pub name: &'static str,
     pub pid: Option<u32>,
     pub phase: Option<String>,
-    pub last_log: Option<std::path::PathBuf>,
 }
 
 /// Persistent state for the agent orchestration cycle.
@@ -164,7 +164,7 @@ impl AgentCycleState {
             .create(true).append(true).open(log_path).ok();
 
         let agents = AGENT_CYCLE_NAMES.iter()
-            .map(|&name| AgentInfo { name, pid: None, phase: None, last_log: None })
+            .map(|&name| AgentInfo { name, pid: None, phase: None })
             .collect();
 
         AgentCycleState {