[tool] octal_to_decimal

[selvakanthanjagavan-byte]

Checklist

• File is in tools/ directory
• File has header comments (# tool, # description, # author, # example)
• File has a run() function that returns a string
• Uses stdlib only (no external dependencies)
• Tested locally with python bitbox.py octal_to_decimal 77

Closes #207

[abduznik]

Hey! Thanks for the contribution. A few things before merging:

1. No error handling — If someone runs or , it will crash. Please add error handling for invalid octal input.
2. Format — Use header format instead of docstring (see other tools in for reference).
3. Missing trailing newline — Add a newline at end of file.

The core logic looks correct. Once these are fixed, good to go!