context: cache role header token lengths

Branch::tokens() was calling tokenizer::encode() on every call for
the role header ("system\n", "user\n", "assistant\n") and trailing
newline. In trim_conversation(), this meant hundreds of encode calls
per trim cycle.

These are fixed strings - cache them with OnceLock on first use.

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

src/agent/context.rs

@@ -31,8 +31,31 @@
 
 use chrono::{DateTime, Utc};
 use serde::{Serialize, Deserialize};
+use std::sync::OnceLock;
 use super::tokenizer;
 
+// Cached token lengths for role headers — computed once on first use.
+// "system\n", "user\n", "assistant\n" and "\n" are fixed strings.
+static ROLE_TOKENS: OnceLock<[usize; 3]> = OnceLock::new();
+static NEWLINE_TOKENS: OnceLock<usize> = OnceLock::new();
+
+fn role_header_tokens(role: Role) -> usize {
+    let tokens = ROLE_TOKENS.get_or_init(|| [
+        tokenizer::encode("system\n").len(),
+        tokenizer::encode("user\n").len(),
+        tokenizer::encode("assistant\n").len(),
+    ]);
+    match role {
+        Role::System => tokens[0],
+        Role::User => tokens[1],
+        Role::Assistant => tokens[2],
+    }
+}
+
+fn newline_tokens() -> usize {
+    *NEWLINE_TOKENS.get_or_init(|| tokenizer::encode("\n").len())
+}
+
 // ---------------------------------------------------------------------------
 // Types
 // ---------------------------------------------------------------------------
@@ -423,9 +446,9 @@ impl Ast for AstNode {
         match self {
             Self::Leaf(leaf) => leaf.tokens(),
             Self::Branch { role, children, .. } => {
-                1 + tokenizer::encode(&format!("{}\n", role.as_str())).len()
+                1 + role_header_tokens(*role)
                     + children.iter().map(|c| c.tokens()).sum::<usize>()
-                    + 1 + tokenizer::encode("\n").len()
+                    + 1 + newline_tokens()
             }
         }
     }