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/dedup.rs

@@ -1,3 +1,4 @@
+#![allow(dead_code)]
 //! Cross-call tool-result deduplication: when a read-only tool is called
 //! again with identical arguments, the earlier result is replaced with a
 //! placeholder so only the latest copy occupies context.