Fast startup: only retokenize tail of conversation log

restore_from_log reads the full log but walks backwards from the tail,
retokenizing each node as it goes. Stops when conversation budget is
full. Only the nodes that fit get pushed into context.

Added AstNode::retokenize() — recomputes token_ids on all leaves
after deserialization (serde skip means they're empty).

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

src/agent/context.rs

@@ -296,6 +296,23 @@ impl AstNode {
 
     // -- Builder --------------------------------------------------------------
 
+    pub fn retokenize(self) -> Self {
+        match self {
+            Self::Leaf(leaf) => {
+                let token_ids = if leaf.body.is_prompt_visible() {
+                    tokenizer::encode(&leaf.body.render())
+                } else {
+                    vec![]
+                };
+                Self::Leaf(NodeLeaf { token_ids, ..leaf })
+            }
+            Self::Branch { role, children } => Self::Branch {
+                role,
+                children: children.into_iter().map(|c| c.retokenize()).collect(),
+            },
+        }
+    }
+
     pub fn with_timestamp(mut self, ts: DateTime<Utc>) -> Self {
         match &mut self {
             Self::Leaf(leaf) => leaf.timestamp = Some(ts),