shared active tools: Agent writes, TUI reads directly

Move active tool tracking from TUI message-passing to shared
Arc<RwLock> state. Agent pushes on dispatch, removes on
apply_tool_result. TUI reads during render. Background tasks
show as active until drained at next turn start.

Co-Developed-By: Kent Overstreet <kent.overstreet@linux.dev>

src/user/ui_channel.rs

@@ -22,6 +22,22 @@ pub fn shared_context_state() -> SharedContextState {
     Arc::new(RwLock::new(Vec::new()))
 }
 
+/// Active tool info for TUI display.
+#[derive(Debug, Clone)]
+pub struct ActiveTool {
+    pub id: String,
+    pub name: String,
+    pub detail: String,
+    pub started: std::time::Instant,
+}
+
+/// Shared active tools — agent writes, TUI reads.
+pub type SharedActiveTools = Arc<RwLock<Vec<ActiveTool>>>;
+
+pub fn shared_active_tools() -> SharedActiveTools {
+    Arc::new(RwLock::new(Vec::new()))
+}
+
 /// Which pane streaming text should go to.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum StreamTarget {