feat(context): tambah context::tokens dengan tiktoken-rs

Ganti tiga heuristik char-count (/3 di shortsend, /4 di loop turn, /4
di status bar) yang saling tidak konsisten dengan satu BPE tokenizer
nyata. tiktoken-rs membundel vocab lewat include_str! saat build, jadi
tidak ada akses jaringan saat runtime.

src/app/runtime/context/mod.rs

@@ -0,0 +1,13 @@
+//! Context management: token counting, cross-call tool-result dedup,
+//! per-result compression, budget-based shaping, and shared
+//! context-window resolution — replaces `runtime::shortsend`.
+//!
+//! No facade function here: `dedup`, `shaping`, and `tokens` are called
+//! directly from each call site (the per-turn auto-compaction loop in
+//! `actions::run_agent_turn`, and `Action::Compact`), matching this
+//! codebase's "no DI, call modules directly" convention. An orchestration
+//! layer would only serve one of the two callers generically — the
+//! auto-loop already needs per-stage control to decide when to emit
+//! `TurnEvent::Compacted`.
+
+pub mod tokens;