Make --character-set an alias for --charset at the CLI

[rolandwalker]

Description

Make --character-set an alias for --charset at the CLI, for alignment with the spelling of the option in ~/.myclirc.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

Findings

• Missing test: No CLI test covers --character-set parsing. Add a small CLI invocation test to ensure --character-set maps to the same runtime charset behavior as --charset (e.g., in test/ for CLI option parsing). This prevents regressions if click options are refactored. mycli/main.py:1713

Notes

• No correctness or security issues spotted in this change.