Add notes and sets support to keep

[OrkoHunter]

Closes https://github.com/OrkoHunter/keep/issues/96

Summary

• add new notes and set save helpers in utils
• convert keep new to a group with notes and set subcommands
• mention new commands in README

Testing

• pip install -e .
• keep new --help
• keep new notes --help
• keep new set --help

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https://chatgpt.com/codex/tasks/task_e_687a3938b5e88323a408459121ab108b