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/mod.rs

@@ -469,6 +469,7 @@ impl Agent {
     ) {
         let mut nodes = Vec::new();
         for (call, output) in &results {
+            if call.name == "yield_to_user" { continue; }
             ds.had_tool_calls = true;
             if output.starts_with("Error:") { ds.tool_errors += 1; }
             nodes.push(Self::make_tool_result_node(call, output));