agent: don't hold agent lock across I/O

The agent lock was held for the entire duration of turn() — including
API streaming and tool dispatch awaits. This blocked the UI thread
whenever it needed the lock (render tick, compaction check, etc.),
causing 20+ second freezes.

Fix: turn() takes Arc<Mutex<Agent>> and manages locking internally.
Lock is held briefly for prepare/process phases, released during all
I/O (streaming, tool awaits, sleep retries). Also:

- check_compaction: spawns task instead of awaiting on event loop
- start_memory_scoring: already spawned, no change needed
- dispatch_tool_call_unlocked: drops lock before tool handle await
- Subconscious screen: renders all agents from state dynamically
  (no more hardcoded SUBCONSCIOUS_AGENTS list)
- Memory scoring shows n/m progress in snapshots

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

src/user/mod.rs

@@ -433,7 +433,7 @@ impl App {
             Screen::Subconscious => {
                 match key.code {
                     KeyCode::Up => { self.agent_selected = self.agent_selected.saturating_sub(1); self.debug_scroll = 0; return; }
-                    KeyCode::Down => { self.agent_selected = (self.agent_selected + 1).min(SUBCONSCIOUS_AGENTS.len() - 1); self.debug_scroll = 0; return; }
+                    KeyCode::Down => { self.agent_selected = (self.agent_selected + 1).min(self.agent_state.len().saturating_sub(1)); self.debug_scroll = 0; return; }
                     KeyCode::Enter | KeyCode::Right => { self.agent_log_view = true; self.debug_scroll = 0; return; }
                     KeyCode::Left | KeyCode::Esc => {
                         if self.agent_log_view { self.agent_log_view = false; self.debug_scroll = 0; }