docs(readme): plain English, bullets and numbered steps

[jshvn]

What

Rewrites the README in plain English. Everything it says is unchanged.

• The bolded one-liners ("Catch-up is plan, not systemd", "At most once, by writing ahead") are now ordinary sentences.
• These are now bullet lists:
  • what it does
  • the slot rules
  • the three workflow requirements
  • the retry rules
  • troubleshooting
• "How it works" is a numbered list of what one tick does, and the diagram uses short plain labels.
• The setup is five numbered steps, where it used to be five sub-headings.
• The commands, the slot table, the guarantees and the Nix snippet are unchanged.

Test plan

• Rendered in the Markdown preview; the mermaid diagram, the table and the code blocks all render
• Every command and path matches the Taskfile and module.nix