save_agent_log: write flat context array matching AST order

The old code wrote a JSON object with named section keys, which
serde_json serialized in alphabetical order — putting conversation
before system, making logs misleading. Write a single flat array
in section order instead, matching what the model actually sees.

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

src/agent/context.rs

@@ -764,7 +764,7 @@ impl ContextState {
     pub fn conversation(&self) -> &[AstNode] { &self.conversation }
     pub fn conversation_mut(&mut self) -> &mut Vec<AstNode> { &mut self.conversation }
 
-    fn sections(&self) -> [&Vec<AstNode>; 4] {
+    pub fn sections(&self) -> [&Vec<AstNode>; 4] {
         [&self.system, &self.identity, &self.journal, &self.conversation]
     }
 }