Preserve truncated output recovery details

[biefan]

Summary

• measure truncation based on visible characters so ANSI escape sequences do not trigger premature truncation
• save the full output to a temp file before truncating it
• replace the broken computer.ai.summarize(result) advice with shell commands users can actually run
• add regression tests for ANSI-heavy output and temp-file recovery

Testing

• uv run --python 3.11 --with pytest python -m pytest tests/core/utils/test_truncate_output.py -q
• python3 -m py_compile interpreter/core/utils/truncate_output.py tests/core/utils/test_truncate_output.py

Closes #1695