Move Subconscious + SubconsciousAgent into dmn.rs

Subconscious owns agents and shared walked state. trigger() and
collect_results() take the conscious agent Arc as a parameter.
Mind holds Subconscious behind a tokio Mutex and calls into it
from the event loop.

Drops ~170 lines from mind/mod.rs.

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

src/user/subconscious.rs

@@ -76,7 +76,7 @@ impl SubconsciousScreen {
         let hint = Style::default().fg(Color::DarkGray).add_modifier(Modifier::ITALIC);
 
         lines.push(Line::raw(""));
-        let walked = app.subconscious_shared.walked.len();
+        let walked = app.walked_count;
         lines.push(Line::styled(
             format!("── Subconscious Agents ──  walked: {}", walked), section));
         lines.push(Line::styled("  (↑/↓ select, Enter view log)", hint));