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.

Cargo.lock

@@ -196,6 +196,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5cee35f73844aa3014bb606320a6c1f010249dbdf43342fe54b5a4f6a8ed4b79"
 dependencies = [
  "memchr",
+ "regex-automata",
  "serde_core",
 ]
 
@@ -807,6 +808,17 @@ dependencies = [
  "regex-syntax",
 ]
 
+[[package]]
+name = "fancy-regex"
+version = "0.17.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72cf461f865c862bb7dc573f643dd6a2b6842f7c30b07882b56bd148cc2761b8"
+dependencies = [
+ "bit-set 0.8.0",
+ "regex-automata",
+ "regex-syntax",
+]
+
 [[package]]
 name = "fast-srgb8"
 version = "1.0.0"
@@ -3521,6 +3533,21 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "tiktoken-rs"
+version = "0.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "027853bbf8c7763b77c5c595f1c271c7d536ced7d6f83452911b944621e57fc2"
+dependencies = [
+ "anyhow",
+ "base64",
+ "bstr",
+ "fancy-regex 0.17.0",
+ "lazy_static",
+ "regex",
+ "rustc-hash",
+]
+
 [[package]]
 name = "time"
 version = "0.3.53"
@@ -4487,6 +4514,7 @@ dependencies = [
  "sha2 0.11.0",
  "similar",
  "syntect",
+ "tiktoken-rs",
  "tokio",
  "tracing",
  "tracing-subscriber",