Delete ui_channel.rs — relocate types, remove all UiMessage/UiSender plumbing

Types relocated:
- StreamTarget → mind/mod.rs (Mind decides Conversation vs Autonomous)
- SharedActiveTools + shared_active_tools() → agent/tools/mod.rs
- ContextSection + SharedContextState → agent/context.rs (already there)
- StatusInfo + ContextInfo → user/mod.rs (UI display state)

Removed UiSender from: Agent::turn, Mind, learn.rs, all function signatures.
The entire message-passing layer is gone. All state flows through
Agent fields (activities, entries, streaming) read by the UI via try_lock.

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

src/agent/tools/mod.rs

@@ -22,6 +22,7 @@ pub mod working_stack;
 
 use std::future::Future;
 use std::pin::Pin;
+use std::sync::Arc;
 use std::time::Instant;
 
 fn default_timeout() -> u64 { 120 }
@@ -70,6 +71,13 @@ pub struct ActiveToolCall {
     pub handle: tokio::task::JoinHandle<(ToolCall, String)>,
 }
 
+/// Shared active tool calls — agent spawns, TUI reads metadata / aborts.
+pub type SharedActiveTools = Arc<std::sync::Mutex<Vec<ActiveToolCall>>>;
+
+pub fn shared_active_tools() -> SharedActiveTools {
+    Arc::new(std::sync::Mutex::new(Vec::new()))
+}
+
 /// Truncate output if it exceeds max length, appending a truncation notice.
 pub fn truncate_output(mut s: String, max: usize) -> String {
     if s.len() > max {