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/user/chat.rs

@@ -16,7 +16,7 @@ use super::{
     App, HotkeyAction, ScreenAction, ScreenView,
     screen_legend,
 };
-use crate::user::ui_channel::StreamTarget;
+use crate::mind::StreamTarget;
 use crate::mind::MindCommand;
 
 // --- Slash command table ---