fix(cli): validate that --cwd directory exists before execution (#4658)

* fix(cli): validate that --cwd directory exists before execution

When a non-existent directory is passed via --cwd/-d, commitlint now
prints a clear error message and exits with code 1 instead of silently
returning exit code 0.

Closes #4595

* fix(cli): validate --cwd is a directory, not just that it exists

Replaces existsSync with statSync + isDirectory to reject file paths
that would cause uncontrolled errors downstream in git operations.

* fix(cli): differentiate --cwd error messages by errno code

The catch-all handler for statSync failures reported all errors
(EACCES, EPERM, ENOTDIR) as "does not exist". Now checks
error.code and only uses that message for ENOENT; other errors
surface the actual errno code.