feat: add Python 3.13 support

[HC-ONLINE]

Summary

• Add Programming Language :: Python :: 3.13 classifier to pyproject.toml
• Add 3.13 to the CI test matrix (.github/workflows/ci.yml)
• Bump Docker base images from python:3.12-slim to python:3.13-slim
• Update supported versions in AGENTS.md

Validation

• uv lock --check: no lockfile changes needed (all pinned deps ship cp313 wheels; cryptography uses cp311-abi3)
• Full test suite on Python 3.13.12: 1515 passed, 3 skipped
• ruff check,
• ruff format --check, pyright: clean
• docker build on 3.13: image runs Python 3.13.16 and ciberwebscan version works