New errors.ts module with ToolError hierarchy:
- ToolTimeoutError (TIMEOUT)  command exceeded timeout
- ToolPermissionError (PERMISSION_DENIED)  safe mode blocked
- ToolNotFoundError (NOT_FOUND)  file/directory missing
- ToolValidationError (VALIDATION)  invalid parameters

ToolResult now has optional errorCode field. All 6 built-in tools
tag errors with codes. classifyError() utility for string-to-code.

10 new tests. Total: 60 tests, 96 expect() calls.
Version bump: 2.1.0  2.2.0