feat(context): tambah context::shaping (port dari shortsend)

Perilaku should_shape/shape_messages tidak berubah, hanya sumber
penghitungan token yang sekarang lewat context::tokens (tiktoken-rs)
menggantikan heuristik char/3 bawaannya sendiri.

src/app/runtime/context/tokens.rs

@@ -28,6 +28,7 @@ pub fn count_tokens(text: &str) -> usize {
 ///
 /// Return: 0 for a message with no `content` (e.g. an assistant message
 /// that only carries `tool_calls`).
+#[allow(dead_code)]
 pub fn count_message_tokens(msg: &ChatMessage) -> usize {
     msg.content.as_deref().map_or(0, count_tokens)
 }