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

Follow up on the ongoing issue #463 and PR #756

Currently, inside #756, 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" on StarsExpress:fix-read-text-sanitized, which is the branch supporting #756, 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

All these will be covered inside a brand new PR once #756 merges into main.

Start by reading PR #756 and the existing implementations in code_puppy/tools/common.py, code_puppy/agents/_runtime.py, and code_puppy/config.py. Inspect the dedicated test in tests/command_line/test_remaining_coverage.py and run the relevant tests before changing the duplicated sanitization paths. Done means the active callers use the shared sanitizer, dead code and its dedicated test are removed if confirmed unused, and surrogate handling remains covered.