Kill tiktoken — all token counting now uses Qwen 3.5 tokenizer

Remove tiktoken-rs dependency, CoreBPE field on Agent, and the
msg_token_count() function. All tokenization now goes through the
global HuggingFace tokenizer in agent/tokenizer.rs.

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

src/agent/context.rs

@@ -7,7 +7,6 @@
 use crate::agent::api::*;
 use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
-use tiktoken_rs::CoreBPE;
 use crate::agent::tools::working_stack;
 
 // --- Context state types ---
@@ -254,26 +253,6 @@ fn lowest_scored_memory(entries: &[ContextEntry]) -> Option<usize> {
         .map(|(i, _)| i)
 }
 
-/// Count the token footprint of a message using BPE tokenization.
-pub fn msg_token_count(tokenizer: &CoreBPE, msg: &Message) -> usize {
-    let count = |s: &str| tokenizer.encode_with_special_tokens(s).len();
-    let content = msg.content.as_ref().map_or(0, |c| match c {
-        MessageContent::Text(s) => count(s),
-        MessageContent::Parts(parts) => parts.iter()
-            .map(|p| match p {
-                ContentPart::Text { text } => count(text),
-                ContentPart::ImageUrl { .. } => 85,
-            })
-            .sum(),
-    });
-    let tools = msg.tool_calls.as_ref().map_or(0, |calls| {
-        calls.iter()
-            .map(|c| count(&c.function.arguments) + count(&c.function.name))
-            .sum()
-    });
-    content + tools
-}
-
 /// Detect context window overflow errors from the API.
 pub fn is_context_overflow(err: &anyhow::Error) -> bool {
     let msg = err.to_string().to_lowercase();