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/context.rs

@@ -13,6 +13,7 @@ use ratatui::{
 };
 
 use super::{App, ScreenAction, ScreenView, screen_legend};
+use crate::agent::context::ContextSection;
 
 pub(crate) struct ConsciousScreen {
     scroll: u16,
@@ -25,12 +26,12 @@ impl ConsciousScreen {
         Self { scroll: 0, selected: None, expanded: std::collections::HashSet::new() }
     }
 
-    fn read_context_state(&self, app: &App) -> Vec<crate::user::ui_channel::ContextSection> {
+    fn read_context_state(&self, app: &App) -> Vec<ContextSection> {
         app.shared_context.read().map_or_else(|_| Vec::new(), |s| s.clone())
     }
 
-    fn item_count(&self, context_state: &[crate::user::ui_channel::ContextSection]) -> usize {
-        fn count_section(section: &crate::user::ui_channel::ContextSection, expanded: &std::collections::HashSet<usize>, idx: &mut usize) -> usize {
+    fn item_count(&self, context_state: &[ContextSection]) -> usize {
+        fn count_section(section: &ContextSection, expanded: &std::collections::HashSet<usize>, idx: &mut usize) -> usize {
             let my_idx = *idx;
             *idx += 1;
             let mut total = 1;
@@ -63,7 +64,7 @@ impl ConsciousScreen {
 
     fn render_section(
         &self,
-        section: &crate::user::ui_channel::ContextSection,
+        section: &ContextSection,
         depth: usize,
         lines: &mut Vec<Line>,
         idx: &mut usize,