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/agent/oneshot.rs

@@ -93,8 +93,7 @@ impl Backend {
         match self {
             Backend::Standalone { messages, .. } => messages.push(msg),
             Backend::Forked(agent) => {
-                agent.lock().await.context.entries.push(
-                    super::context::ConversationEntry::Message(msg));
+                agent.lock().await.push_message(msg);
             }
         }
     }