Move conversation_log from AgentState to ContextState

The log records what goes into context, so it belongs under the context
lock. push() now auto-logs conversation entries, eliminating all the
manual lock-state-for-log, drop, lock-context-for-push dances.

- ContextState: new conversation_log field, Clone impl drops it
  (forked contexts don't log)
- push(): auto-logs Section::Conversation entries
- push_node, apply_tool_results, collect_results: all simplified
- collect_results: batch nodes under single context lock
- Assistant response logged under context lock after parse completes

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

src/agent/context.rs

@@ -99,12 +99,24 @@ pub enum AstNode {
 /// The context window: four sections as Vec<AstNode>.
 /// All mutation goes through ContextState methods to maintain the invariant
 /// that token_ids on every leaf matches its rendered text.
-#[derive(Clone)]
 pub struct ContextState {
     system: Vec<AstNode>,
     identity: Vec<AstNode>,
     journal: Vec<AstNode>,
     conversation: Vec<AstNode>,
+    pub conversation_log: Option<crate::mind::log::ConversationLog>,
+}
+
+impl Clone for ContextState {
+    fn clone(&self) -> Self {
+        Self {
+            system: self.system.clone(),
+            identity: self.identity.clone(),
+            journal: self.journal.clone(),
+            conversation: self.conversation.clone(),
+            conversation_log: None, // forked contexts don't log
+        }
+    }
 }
 
 /// Identifies a section for mutation methods.
@@ -698,6 +710,7 @@ impl ContextState {
             identity: Vec::new(),
             journal: Vec::new(),
             conversation: Vec::new(),
+            conversation_log: None,
         }
     }
 
@@ -753,6 +766,13 @@ impl ContextState {
     }
 
     pub fn push(&mut self, section: Section, node: AstNode) {
+        if section == Section::Conversation {
+            if let Some(ref log) = self.conversation_log {
+                if let Err(e) = log.append_node(&node) {
+                    eprintln!("warning: log: {:#}", e);
+                }
+            }
+        }
         self.section_mut(section).push(node);
     }