fix: handle encoding in load_gitignores (issue #2888)

[chrislazar25]

Summary: Fix UnicodeDecodeError when loading .gitignore files containing UTF-8 non-ASCII characters on Windows by specifying encoding='utf-8'.

Technical Details:

• Root Cause: load_gitignores() in watch.py opens .gitignore with plain open(path), letting Python fall back to the OS default encoding (cp1252 on Windows). A byte like 0x9d in a UTF-8 Cyrillic comment triggers UnicodeDecodeError.
• Mechanism: .gitignore files are conventionally UTF-8. Adding encoding='utf-8', errors='replace' matches convention and safely handles edge-case bytes that would be non-UTF-8.

Verification: Confirmed adherence to CONTRIBUTING.md. Ran python -m pytest tests/ -x -q. 455 passed, 1 failed (test_voice_init_invalid_format pre-existing, requires audio hardware). Pre-commit hooks (isort, black, flake8, codespell) all green.

Blast Radius:

• Impact: Low. Only affects .gitignore file reads in the FileWatcher init path.
• Trade-offs: errors='replace' substitutes replacement characters (�) for genuinely non-UTF-8 bytes, which could only occur in comments. ignore patterns are ASCII-safe.

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Fixes #2888