Additional surrogate-sanitizing file read outside `code_puppy/tools`.

[StarsExpress]

Follow up on merged PR #756

Inside main, code_puppy/tools/common.py::_sanitize_string is implemented like this:

def _sanitize_string(text: str) -> str:
    """Sanitize a string to remove invalid Unicode surrogates.
    This handles encoding issues common on Windows with copy-paste operations.
    """
    if not text:
        return text

    try:
        # Try encoding — if it works, string is clean.
        text.encode("utf-8")
        return text

    except UnicodeEncodeError:
        pass

    try:
        # Encode allowing surrogates, then decode replacing them.
        return text.encode("utf-8", errors="surrogatepass").decode(
            "utf-8", errors="replace"
        )

    except (UnicodeEncodeError, UnicodeDecodeError):
        # Last resort: filter out surrogate characters.
        return "".join(
            char if ord(char) < 0xD800 or ord(char) > 0xDFFF else "\ufffd"
            for char in text
        )

Additional duplications

I ran rg "surrogatepass", and found these:

1. code_puppy/command_line/completers.py::_sanitize_for_encoding

Appears to be dead code in production; its only reference is tests/command_line/test_remaining_coverage.py::test_sanitize_for_encoding_unicode_error, which exists solely for test coverage.

Candidates for removal are this function + its dedicated test.

2. code_puppy/agents/_runtime.py::_sanitize_prompt

Actively used with duplicated surrogate-stripping logic as code_puppy/tools/common.py::_sanitize_string.

Consolidation candidate here — call _sanitize_string via a thin wrapper. Like this:

from code_puppy.tools.common import _sanitize_string

def _sanitize_prompt(prompt: str) -> str:
    """Strip lone UTF-16 surrogates (common on Windows copy-paste)."""
    return _sanitize_string(prompt)

3. code_puppy/config.py::normalize_command_history

Consolidation candidate here — call _sanitize_string.

4. code_puppy/config.py::save_command_to_history

Consolidation candidate here — call _sanitize_string.

Plan

Will open a PR to cover them all.