Add contributor guide and update Python tooling

[ileodo]

Summary

• Add AGENTS.md with repo layout, commands, style, test rules, and PR guidance.
• Make local make targets use .venv by default, accept an active or named environment, and check for Python 3.10 or newer. New environments use Python 3.14 by default.
• Run CI on Python 3.13 and 3.14. Keep the package requirement at Python 3.10 or newer and set Ruff's syntax target to Python 3.10.

Checks

• Python 3.10 unit tests: 26 passed.
• Python 3.14 unit tests: 26 passed.
• Ruff lint: passed.
• Ruff format check: passed.
• Mypy: passed.
• git diff --check: passed.

Integration tests need MONEYWIZ_TEST_DB_PATH and were not run.