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/user/mod.rs

@@ -19,7 +19,7 @@ use crate::user::{self as tui};
 // --- TUI infrastructure (moved from tui/mod.rs) ---
 
 use ratatui::crossterm::{
-    event::{EnableMouseCapture, DisableMouseCapture},
+    event::{EnableMouseCapture, DisableMouseCapture, EnableBracketedPaste, DisableBracketedPaste},
     terminal::{self, EnterAlternateScreen, LeaveAlternateScreen},
     ExecutableCommand,
 };
@@ -98,6 +98,7 @@ struct ChannelStatus {
 struct App {
     status: StatusInfo,
     activity: String,
+    activity_started: Option<std::time::Instant>,
     running_processes: u32,
     reasoning_effort: String,
     temperature: f32,
@@ -125,6 +126,7 @@ impl App {
                 turn_tools: 0, context_budget: String::new(),
             },
             activity: String::new(),
+            activity_started: None,
             running_processes: 0,
             reasoning_effort: "none".to_string(),
             temperature: 0.6,
@@ -164,12 +166,14 @@ fn init_terminal() -> io::Result<ratatui::Terminal<CrosstermBackend<io::Stdout>>
     let mut stdout = io::stdout();
     stdout.execute(EnterAlternateScreen)?;
     stdout.execute(EnableMouseCapture)?;
+    stdout.execute(EnableBracketedPaste)?;
     let backend = CrosstermBackend::new(stdout);
     ratatui::Terminal::new(backend)
 }
 
 fn restore_terminal(terminal: &mut ratatui::Terminal<CrosstermBackend<io::Stdout>>) -> io::Result<()> {
     terminal::disable_raw_mode()?;
+    terminal.backend_mut().execute(DisableBracketedPaste)?;
     terminal.backend_mut().execute(DisableMouseCapture)?;
     terminal.backend_mut().execute(LeaveAlternateScreen)?;
     terminal.show_cursor()
@@ -319,7 +323,7 @@ async fn run(
     let (event_tx, mut event_rx) = tokio::sync::mpsc::unbounded_channel();
     std::thread::spawn(move || {
         loop {
-            match crossterm::event::read() {
+            match ratatui::crossterm::event::read() {
                 Ok(event) => { if event_tx.send(event).is_err() { break; } }
                 Err(_) => break,
             }