Mouse selection, copy/paste, yield_to_user fixes

- Mouse text selection with highlight rendering in panes
- OSC 52 clipboard copy on selection, middle-click paste via tmux buffer
- Bracketed paste support (Event::Paste)
- yield_to_user: no tool result appended, ends turn immediately
- yield_to_user: no parameters, just a control signal
- Drop arboard dependency, use crossterm OSC 52 + tmux for clipboard

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

src/agent/tools/control.rs

@@ -33,14 +33,12 @@ pub(super) fn tools() -> [super::Tool; 3] {
                })) },
         Tool { name: "yield_to_user",
                description: "Wait for user input before continuing. The only way to enter a waiting state.",
-               parameters_json: r#"{"type":"object","properties":{"message":{"type":"string","description":"Optional status message"}}}"#,
-               handler: Arc::new(|agent, v| Box::pin(async move {
-                   let msg = v.get("message").and_then(|v| v.as_str()).unwrap_or("Waiting for input.");
+               parameters_json: r#"{"type":"object","properties":{}}"#,
+               handler: Arc::new(|agent, _| Box::pin(async move {
                    if let Some(agent) = agent {
-                       let mut a = agent.state.lock().await;
-                       a.pending_yield = true;
+                       agent.state.lock().await.pending_yield = true;
                    }
-                   Ok(format!("Yielding. {}", msg))
+                   Ok(String::new())
                })) },
     ]
 }