fix: address bot review — surrogate-pair counting and CJK line splitting

- Use code-point length instead of UTF-16 length in estimateStringChars()
  so that CJK Extension B+ surrogate pairs (U+20000+) are counted as 1
  character, not 2 (fixes ~25% overestimate for rare characters).

- Change long-line split step from maxChars to chunking.tokens so that
  CJK lines are sliced into token-budget-sized segments instead of
  char-budget-sized segments that produce ~4x oversized chunks.

- Add tests for both fixes: surrogate-pair handling and long CJK line
  splitting.

Addresses review feedback from Greptile and Codex bots.

src/utils/cjk-chars.test.ts

@@ -81,7 +81,6 @@ describe("estimateStringChars", () => {
     // "你" counts as 4, emoji remains 2 => total 6
     expect(estimateStringChars("你😀")).toBe(6);
   });
-
   it("yields ~1 token per CJK char when divided by CHARS_PER_TOKEN_ESTIMATE", () => {
     // 10 CJK chars should estimate as ~10 tokens
     const cjk = "这是一个测试用的句子呢";