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/api/openai.rs

@@ -9,7 +9,6 @@ use reqwest::Client;
 use tokio::sync::mpsc;
 
 use super::types::*;
-use crate::user::ui_channel::UiSender;
 use super::StreamEvent;
 
 /// Stream SSE events from an OpenAI-compatible endpoint, sending
@@ -23,7 +22,6 @@ pub(super) async fn stream_events(
     messages: &[Message],
     tools_json: &serde_json::Value,
     tx: &mpsc::UnboundedSender<StreamEvent>,
-    ui_tx: &UiSender,
     reasoning_effort: &str,
     sampling: super::SamplingParams,
     priority: Option<i32>,
@@ -71,7 +69,7 @@ pub(super) async fn stream_events(
     )
     .await?;
 
-    let mut reader = super::SseReader::new(ui_tx);
+    let mut reader = super::SseReader::new();
     reader.request_json = request_json;
 
     let mut content_len: usize = 0;
@@ -167,7 +165,6 @@ pub(super) async fn stream_events(
     let total_elapsed = reader.stream_start.elapsed();
 
     super::log_diagnostics(
-        ui_tx,
         content_len,
         tool_call_count,
         reasoning_chars,