docs: revamp the README — less install text, a "What's cool" section #572

Summary

The README is long and repeats install steps that the install buttons already link to. We make it shorter and show what the plugin does with pictures.

What we found

• The README has codex and /plugin install commands. The install buttons in the table already open the full steps for each path.
• The Claude Code plugin section has two long command tables and a save-more inventory. A new reader must read a lot before they see why the plugin is useful.

Proposal

1. Remove the install commands from the README (Codex marketplace add block and the Claude Code /plugin block). The buttons stay.
2. Rename "Claude Code plugin" to "What's cool", and show:
   • a screenshot of the status line with subagents, from a real session;
   • a screenshot of insights;
   • three links, each to a page with a screenshot: save-more, remove tools, dashboard.
3. Move all the command tables and the save-more inventory to a dedicated page (docs/plugin/commands.md).
4. The rest of the README stays the same.

Open questions

• Screenshots for save-more, remove tools and the dashboard come from Osher.