[Bug] docx skill: validate.py and pack.py crash on Windows due to missing UTF-8 encoding in file I/O and print output

## Description

The docx skill's `validate.py` and `pack.py` scripts fail on Windows with `UnicodeEncodeError` / `UnicodeDecodeError` because they rely on the default system encoding (cp1252 on Windows) instead of explicitly using UTF-8.

There are two independent failure modes:

### 1. Output encoding crash (print statements with Unicode characters)

`validators/docx.py` lines 249 and 431 use the `→` (U+2192) arrow character in print statements:

```python
# Line 249
print(f"\nParagraphs: {original_count} → {new_count} ({diff_str})")
# Line 431
f" Repaired: {xml_file.name}: durableId {durable_id} → {new_id}"
```

On Windows (cp1252 default encoding), this crashes with:
```
UnicodeEncodeError: 'charmap' codec can't encode character '\u2192' in position 17: character maps to
```

### 2. Input encoding crash (reading XML files)

`validators/base.py` line 763 opens XML files without specifying encoding:

```python
with open(xml_file, "r") as f:
xml_doc = lxml.etree.parse(f)
```

When an XML file contains bytes outside the cp1252 range (e.g., `numbering.xml` generated by docx-js), this crashes with:
```
UnicodeDecodeError: 'charmap' codec can't decode byte 0x8f in position 1410: character maps to
```

## Expected behavior

Scripts should work on Windows without requiring `PYTHONIOENCODING` to be set. XML files should be read as UTF-8 (per the XML spec).

## Suggested fix

1. **Line 763 of `validators/base.py`**: Add `encoding="utf-8"` to the `open()` call:
```python
with open(xml_file, "r", encoding="utf-8") as f:
```

2. **Lines 249, 431 of `validators/docx.py`**: Either replace `→` with `->`, or add `encoding="utf-8"` to any file-based output. (The `PYTHONIOENCODING=utf-8` env var workaround fixes this for terminal output, but the root cause is using non-ASCII in print statements without ensuring the output stream supports it.)

## Workarounds

- Set `PYTHONIOENCODING=utf-8` in the environment (fixes issue 1)
- Use `--validate false` on `pack.py` (bypasses issue 2)

## Environment

- OS: Windows 11 Enterprise (Git Bash)
- Python: 3.x (Anaconda, skill-docx conda env)
- Skills repo commit: `b0cbd3d`
- Document created with: docx-js (Node.js)