Wire executable I/O through ShellIo

[cdprice02]

The execute_executable function in src/executor.rs currently inherits the parent process's stdio handles (noted by the TODO comment on line 107). This means external command output bypasses the ShellIo abstraction entirely, breaking testability and violating ferrish's I/O design.

Expected behavior: Child process stdout and stderr must be piped through the ShellIo trait so that MockIo captures output in tests and the abstraction is consistent across builtins and executables.

Acceptance criteria:

• External command stdout is routed through ShellIo::out_writer()
• External command stderr is routed through ShellIo::err_writer()
• Integration tests can capture external command output via MockIo
• The TODO comment in executor.rs is resolved