fix(sync-regression): restore code-point-safe truncate + ignore node:test suites

Two regressions from the 2026-04-26 agent-core sync (PR #350):

1. lib/cross-platform/index.js::truncate lost its code-point-safe
   slicing (splitting surrogate pairs on emoji) and its non-positive
   maxLength special case. Restored [...text] spread + maxLength <= 0
   early return. Regression tests in __tests__/cross-platform.test.js
   now pass.

2. jest.config.js testMatch picked up lib/binary/index.test.js which is
   a node:test suite (agent-core convention for co-located tests), and
   Jest failed with "must contain at least one test". Added
   /lib/binary/.*\.test\.js$ to testPathIgnorePatterns; run it via
   `node --test lib/binary/*.test.js` instead.

Both fixes should be upstreamed to agent-core so every consumer gets
them uniformly on next sync.

Author: avifenesh

jest.config.js

@@ -9,7 +9,12 @@ module.exports = {
     '/node_modules/',
     '/plugins/.*/lib/',
     '/.claude/worktrees/',
-    '/worktrees/'
+    '/worktrees/',
+    // lib/binary/index.test.js is a node:test suite (co-located with the
+    // module in agent-core convention). Jest's discovery picks it up and
+    // fails with "must contain at least one test" because it has no
+    // describe/it. Run it separately via `node --test lib/binary/*.test.js`.
+    '/lib/binary/.*\\.test\\.js$'
   ],
   modulePathIgnorePatterns: [
     '/.claude/worktrees/',

lib/cross-platform/index.js

@@ -303,15 +303,21 @@ function formatSection(title, content) {
  */
 
 /**
- * Truncate text to limit with ellipsis
+ * Truncate text to limit with ellipsis.
+ *
+ * Slices on Unicode code points (not UTF-16 code units) so multi-byte
+ * chars like emoji never end up as orphan surrogates. Non-positive
+ * maxLength returns the original string unchanged.
  *
  * @param {string} text - Text to truncate
- * @param {number} maxLength - Maximum length
+ * @param {number} maxLength - Maximum length (in code points)
  * @returns {string} Truncated text
  */
 function truncate(text, maxLength) {
-  if (text.length <= maxLength) return text;
-  return text.substring(0, maxLength - 3) + '...';
+  if (maxLength <= 0) return text;
+  const codePoints = [...text];
+  if (codePoints.length <= maxLength) return text;
+  return codePoints.slice(0, maxLength - 3).join('') + '...';
 }
 
 /**