validation(scaffold,companyos): reject regular-file target with clear message

[CryptoJones]

5bc304a validation: catch NotADirectoryError in _cmd_companyos too (#cli)

Iter 12 added NotADirectoryError to scaffold_companyos but
_cmd_companyos only caught FileExistsError, so passing a regular
file path crashed with a stacktrace instead of returning 2 + a
clean error message. Catch both errors in the same except clause.

Test added: socrates companyos returns 2 with 'error:' on
stderr and leaves the file untouched.

051e424 validation(scaffold,companyos): reject regular-file target with clear message

If an operator typo'd
socrates init my-project --base ~/notes.txt
or otherwise passed a path that exists as a regular file, scaffold()
and scaffold_companyos() previously hit confusing late-stage errors:

• scaffold() got "NotADirectoryError: [Errno 20] Not a directory" from
  inside the FILES touch loop — after target.mkdir(exist_ok=overwrite)
  silently no-op'd on the existing-as-file path.
• scaffold_companyos() blew up inside any(target.iterdir()) because
  Path.iterdir on a file raises NotADirectoryError mid-condition.

Both now check target.is_file() up-front and raise a clean
NotADirectoryError with an actionable message. The file is untouched.

Tests added (3):

• scaffold() rejects file target — no side effects
• scaffold(file, overwrite=True) STILL rejects (overwrite is for
  empty dirs, not for silently clobbering user files)
• scaffold_companyos() symmetric guard

150/150 tests pass; ruff + mypy clean.

This branch has TWO commits:

1. The original scaffold/companyos fix (NotADirectoryError on file target).
2. A follow-up that catches NotADirectoryError in _cmd_companyos too — without it, the CLI would have crashed with a stacktrace despite the library function now raising cleanly.