Add serde derives to AST types, enable chrono serde feature

Prep for wiring context_new.rs into the codebase: AstNode, NodeLeaf,
NodeBody, Role all derive Serialize/Deserialize for conversation log
persistence.

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

src/agent/context_new.rs

@@ -30,6 +30,7 @@
 // the NodeBody variant. Grammar constraints enforced by construction.
 
 use chrono::{DateTime, Utc};
+use serde::{Serialize, Deserialize};
 use super::tokenizer;
 
 // ---------------------------------------------------------------------------
@@ -37,7 +38,7 @@ use super::tokenizer;
 // ---------------------------------------------------------------------------
 
 /// Branch roles — maps directly to the grammar's message roles.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 pub enum Role {
     System,
     User,
@@ -45,7 +46,7 @@ pub enum Role {
 }
 
 /// Leaf content — each variant knows how to render itself.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum NodeBody {
     // Children of message branches — rendered without im_start/im_end
     Content(String),
@@ -62,7 +63,7 @@ pub enum NodeBody {
 }
 
 /// A leaf node: typed content with cached token IDs.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct NodeLeaf {
     body: NodeBody,
     token_ids: Vec<u32>,
@@ -70,7 +71,7 @@ pub struct NodeLeaf {
 }
 
 /// A node in the context AST.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub enum AstNode {
     Leaf(NodeLeaf),
     Branch { role: Role, children: Vec<AstNode> },