Replace token counting with token generation via HuggingFace tokenizer

Add agent/tokenizer.rs with global Qwen 3.5 tokenizer that generates
actual token IDs including chat template wrapping. ContextEntry now
stores token_ids: Vec<u32> instead of tokens: usize — the count is
derived from the length.

ContextEntry::new() tokenizes automatically via the global tokenizer.
ContextSection::push_entry() takes a raw ConversationEntry and
tokenizes it. set_message() re-tokenizes without needing an external
tokenizer parameter.

Token IDs include the full chat template: <|im_start|>role\ncontent
<|im_end|>\n — so concatenating token_ids across entries produces a
ready-to-send prompt for vLLM's /v1/completions endpoint.

The old tiktoken CoreBPE is now unused on Agent (will be removed in
a followup). Token counts are now exact for Qwen 3.5 instead of the
~85-90% approximation from cl100k_base.

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

.claude/scheduled_tasks.lock

@@ -0,0 +1 @@
+{"sessionId":"463c6050-b49f-4509-9d4b-4596af79a90e","pid":11339,"acquiredAt":1775649730868}