fix: read gitignore files as utf-8

[Kcstring]

Summary

• Read watcher gitignore files with explicit UTF-8 decoding instead of the platform default encoding
• Use replacement for malformed bytes so file watching does not crash on decode errors
• Add a regression test that simulates a Windows cp1252 default while reading a UTF-8 gitignore

Testing

• python3 -m pytest tests/basic/test_watch.py
• python3 -m ruff check aider/watch.py tests/basic/test_watch.py

Fixes #2888