fix: handle None sys.stdout.encoding in CLI output

[octo-patch]

Fixes #1597

Problem

When sys.stdout.encoding is None (e.g. when stdout is replaced with a
raw binary stream, or in certain CI/redirected environments), the current
_handle_output code in __main__.py calls:

result.markdown.encode(sys.stdout.encoding, errors="replace")

This raises TypeError: encode() argument 'encoding' must be str, not None
instead of the expected UnicodeEncodeError, making the CLI crash with an
unhelpful error.

Solution

Fall back to "utf-8" when sys.stdout.encoding is None:

encoding = sys.stdout.encoding or "utf-8"
print(result.markdown.encode(encoding, errors="replace").decode(encoding))

The existing errors="replace" handling is preserved, so non-UTF-8
terminals (e.g. Windows cp1252) still work correctly.

Testing

Manually verified that the existing errors="replace" path continues to
work, and that the None fallback is triggered correctly:

import sys
enc = None
# Before: raises TypeError
# After: falls back to utf-8 safely
encoding = enc or "utf-8"
print("test \uff89".encode(encoding, errors="replace").decode(encoding))