fix(recall): truncate recalled context by code point, not code unit (#109)

truncateRecallLine capped lines with line.length and line.slice, which
count and index by UTF-16 code unit. When recall.maxCharsPerMemory or
recall.maxTotalRecallChars is set and the cut lands between the halves of
a surrogate pair, the line keeps a lone surrogate that becomes U+FFFD once
UTF-8 encoded for the request, corrupting any non-BMP character (emoji,
CJK Ext-B) in the injected context. Slice on Array.from(line) code points
instead. Both recall call sites route through this function. Same class as
the sanitizeText fix in #31, reintroduced by the #71 budget path.

Signed-off-by: Akhilesh Arora <akhildawra@gmail.com>

src/core/hooks/auto-recall.ts

@@ -776,11 +776,15 @@ function normalizeBudgetLimit(value: number | undefined): number | undefined {
 }
 
 function truncateRecallLine(line: string, maxChars: number): string {
-  if (line.length <= maxChars) return line;
+  // Count and slice by code point, not UTF-16 code unit, so a cut never lands
+  // between the halves of a surrogate pair (which would corrupt a non-BMP
+  // character to U+FFFD when the line is UTF-8 encoded for the request).
+  const cps = Array.from(line);
+  if (cps.length <= maxChars) return line;
   if (maxChars <= RECALL_TRUNCATION_SUFFIX.length) {
-    return line.slice(0, maxChars);
+    return cps.slice(0, maxChars).join("");
   }
-  return `${line.slice(0, maxChars - RECALL_TRUNCATION_SUFFIX.length).trimEnd()}${RECALL_TRUNCATION_SUFFIX}`;
+  return `${cps.slice(0, maxChars - RECALL_TRUNCATION_SUFFIX.length).join("").trimEnd()}${RECALL_TRUNCATION_SUFFIX}`;
 }
 
 /**