Polish hottubctl for public use

[cnberry]

Summary

• add matching hero art, README structure, operations/protocol/troubleshooting/roadmap docs, MIT license, security policy, and CI
• move passwords out of the public config example with HOTTUBCTL_PASSWORD support
• require --yes for temperature writes, validate units, and close failed login sessions
• align with switchctl and the shared language-neutral script/install contract
• declare the Python 3.13+ requirement imposed by python-smarttub 0.0.47

Verification

• 7 unit tests
• Ruff format and lint
• detect-secrets baseline scan
• installer syntax and executable checks
• wheel build, CLI help, Markdown link, and current-tree privacy checks