Add ConversationEntry::Thinking — 0 tokens, not sent to API

Thinking/reasoning content is now a first-class entry type:
- Serialized as {"thinking": "..."} in conversation log
- 0 tokens for budgeting (doesn't count against context window)
- Filtered from assemble_api_messages (not sent back to model)
- Displayed in UI with "thinking: ..." label and expandable content

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

src/user/widgets.rs

@@ -8,7 +8,7 @@ use ratatui::{
     Frame,
     crossterm::event::KeyCode,
 };
-use crate::agent::context::ContextSection;
+use crate::agent::context::{ContextSection, ConversationEntry};
 
 /// UI-only tree node for the section tree display.
 /// Built from ContextSection data; not used for budgeting.
@@ -26,10 +26,10 @@ pub struct SectionView {
 /// Each entry becomes a child with label + expandable content.
 pub fn section_to_view(section: &ContextSection) -> SectionView {
     let children: Vec<SectionView> = section.entries().iter().map(|ce| {
-        let content = if ce.entry.is_log() {
-            String::new()
-        } else {
-            ce.entry.message().content_text().to_string()
+        let content = match &ce.entry {
+            ConversationEntry::Log(_) => String::new(),
+            ConversationEntry::Thinking(text) => text.clone(),
+            _ => ce.entry.message().content_text().to_string(),
         };
         SectionView {
             name: ce.entry.label(),