fix: harden utils.js edge cases and add input validation

- Guard findFiles() against null/undefined dir and pattern parameters
  (previously crashed with TypeError on .replace() or fs.existsSync())
- Wrap countInFile() and grepFile() regex construction in try-catch to
  handle invalid regex strings like '(unclosed' (previously crashed with
  SyntaxError: Invalid regular expression)
- Add try-catch to replaceInFile() with descriptive error logging
- Add 1MB size limit to readStdinJson() matching the PostToolUse hooks
  (previously had unbounded stdin accumulation)
- Improve ensureDir() error message to include the directory path
- Add 128-char length limit to setAlias() to prevent oversized alias
  names from inflating the JSON store
- Update utils.d.ts with new maxSize option on ReadStdinJsonOptions

scripts/lib/utils.d.ts

@@ -123,6 +123,11 @@ export interface ReadStdinJsonOptions {
    * if stdin never closes. Default: 5000
    */
   timeoutMs?: number;
+  /**
+   * Maximum stdin data size in bytes. Prevents unbounded memory growth.
+   * Default: 1048576 (1MB)
+   */
+  maxSize?: number;
 }
 
 /**