fix : handle non-UTF-8 output in subprocess execution

[glasscatya]

Related Issue

Fixes #60

Modification

Use errors="replace" when decoding stdout/stderr in run_bash to prevent UnicodeDecodeError.

Tests

• Existing tests: All just test passed
• Formatting check: just check passed
• Manual verification: GBK-encoded output no longer triggers UnicodeDecodeError in Windows + WSL

Code Quality Checks

• ruff check . passed
• pytest all tests passed

[glasscatya]

Additional Notes

This change aligns with the existing web.fetch utility in the project (see builtin.py line 269),
consistently using errors="replace" to handle potential non-UTF-8 external input.