thalamus: interactive sampling parameter controls

F5 screen now shows temperature, top_p, top_k with interactive
adjustment:
- Up/down: select parameter
- Left/right: adjust value (0.05 steps for temp/top_p, 5 for top_k)
- Updates Agent and display immediately via HotkeyAction

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

src/user/thalamus.rs

@@ -48,12 +48,23 @@ impl App {
         }
         lines.push(Line::raw(""));
 
-        // Sampling parameters
-        lines.push(Line::styled("── Sampling ──", section));
+        // Sampling parameters (↑/↓ select, ←/→ adjust)
+        lines.push(Line::styled("── Sampling (←/→ adjust) ──", section));
         lines.push(Line::raw(""));
-        lines.push(Line::raw(format!("  temperature: {:.2}", self.temperature)));
-        lines.push(Line::raw(format!("  top_p:       {:.2}", self.top_p)));
-        lines.push(Line::raw(format!("  top_k:       {}", self.top_k)));
+        let params = [
+            format!("temperature: {:.2}", self.temperature),
+            format!("top_p:       {:.2}", self.top_p),
+            format!("top_k:       {}", self.top_k),
+        ];
+        for (i, label) in params.iter().enumerate() {
+            let prefix = if i == self.sampling_selected { "▸ " } else { "  " };
+            let style = if i == self.sampling_selected {
+                Style::default().fg(Color::Cyan)
+            } else {
+                Style::default()
+            };
+            lines.push(Line::styled(format!("{}{}", prefix, label), style));
+        }
         lines.push(Line::raw(""));
 
         // Channel status from cached data