feat: add 5 new tools (decimal_to_hex, hex_to_decimal, gcd, factorial, octal_to_decimal)

[chfr19820610-cell]

Summary

This PR adds 5 new mathematical/conversion tools to bitbox:

• decimal_to_hex — Converts decimal integer to hexadecimal string
• hex_to_decimal — Converts hexadecimal string to decimal integer
• gcd — Finds the greatest common divisor of two integers
• factorial — Computes factorial of a non-negative integer
• octal_to_decimal — Converts octal string to decimal integer

Each tool follows the existing pattern: single file in tools/, header comment with metadata, and a run(*args) -> str function.

Closes #193, Closes #194, Closes #195, Closes #196, Closes #197, Closes #198

---

If you have other issues that need fixing or features you'd like added, I'm available for more open source work. Tips appreciated: 1JDmYgWRJipA5ZHDmoVPfpfW8n3Gtbb92c

[abduznik]

Hey! Thanks for the contribution. Before merging, please fix the PR title format:

Change: eat: add 5 new tools (decimal_to_hex, hex_to_decimal, gcd, factorial, octal_to_decimal)
To: [tool] decimal_to_hex, hex_to_decimal, gcd, factorial, octal_to_decimal

Also, please note that some of these tools may already have open PRs or issues. Please check before submitting multiple tools in one PR.

Once fixed, we can review!