WIP: ContextEntry/ContextSection data structures for incremental token counting

New types — not yet wired to callers:

- ContextEntry: wraps ConversationEntry with cached token count and
  timestamp
- ContextSection: named group of entries with cached token total.
  Private entries/tokens, read via entries()/tokens().
  Mutation via push(entry), set(index, entry), del(index).
- ContextState: system/identity/journal/conversation sections + working_stack
- ConversationEntry::System variant for system prompt entries

Token counting happens once at push time. Sections maintain their
totals incrementally via push/set/del. No more recomputing from
scratch on every budget check.

Does not compile — callers need updating.

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

src/user/subconscious.rs

@@ -16,7 +16,8 @@ use ratatui::{
 
 use super::{App, ScreenView, screen_legend};
 use super::widgets::{SectionTree, pane_block_focused, render_scrollable, tree_legend, format_age, format_ts_age};
-use crate::agent::context::ContextSection;
+use crate::agent::context::{ContextSection, ContextEntry, ConversationEntry};
+use crate::agent::api::Message;
 
 #[derive(Clone, Copy, PartialEq)]
 enum Pane { Agents, Outputs, History, Context }
@@ -135,12 +136,13 @@ impl SubconsciousScreen {
             None => return Vec::new(),
         };
         snap.state.iter().map(|(key, val)| {
-            ContextSection {
-                name: key.clone(),
+            let mut section = ContextSection::new(key.clone());
+            section.push(ContextEntry {
+                entry: ConversationEntry::Message(Message::user(val)),
                 tokens: 0,
-                content: val.clone(),
-                children: Vec::new(),
-            }
+                timestamp: None,
+            });
+            section
         }).collect()
     }
 
@@ -151,7 +153,15 @@ impl SubconsciousScreen {
         };
         snap.forked_agent.as_ref()
             .and_then(|agent| agent.try_lock().ok())
-            .map(|ag| ag.conversation_sections_from(snap.fork_point))
+            .map(|ag| {
+                // Build a single section from the forked conversation entries
+                let entries = ag.conversation_entries_from(snap.fork_point);
+                let mut section = ContextSection::new("Conversation");
+                for e in entries {
+                    section.push(e.clone());
+                }
+                vec![section]
+            })
             .unwrap_or_default()
     }
 
@@ -172,7 +182,7 @@ impl SubconsciousScreen {
                     .unwrap_or_else(|| "—".to_string());
                 let entries = snap.forked_agent.as_ref()
                     .and_then(|a| a.try_lock().ok())
-                    .map(|ag| ag.context.entries.len().saturating_sub(snap.fork_point))
+                    .map(|ag| ag.context.conversation.len().saturating_sub(snap.fork_point))
                     .unwrap_or(0);
                 ListItem::from(Line::from(vec![
                     Span::styled(&snap.name, Style::default().fg(Color::Gray)),