validateOutputPath, validateReadPath, and SAFE_DIRECTORIES were duplicated
across write-commands.ts, meta-commands.ts, and read-commands.ts. Extract
to a single shared module with re-exports for backward compatibility.
Also adds validateTempPath() for the upcoming GET /file endpoint (TEMP_DIR
only, not cwd, to prevent remote agents from reading project files).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>