WIP: Context AST design — AstNode with Leaf{text,token_ids}/Branch

New context_new.rs with the AST-based context window design:
- AstNode: role + NodeBody (Leaf with text+token_ids, or Branch with children)
- Tokens only on leaves, branches walk children
- render() produces UTF-8, tokenize produces token IDs, same path
- ResponseParser state machine for streaming assistant responses
- Role enum covers all node types including sections

Still needs: fix remaining pattern match issues, add ContextState wrapper,
wire into mod.rs, replace old context.rs.

Does not compile yet — this is a design checkpoint.

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

src/agent/mod.rs

@@ -15,6 +15,7 @@
 
 pub mod api;
 pub mod context;
+pub mod context_new;
 pub mod oneshot;
 pub mod tokenizer;
 pub mod tools;