docs: restore agent-assisted setup

[quantk]

What changed

• restore the recommended agent-assisted setup block in README
• point the copyable prompt at the renamed herdr-review repository
• make agent-guide inspect existing plugin state and keybindings first
• avoid duplicate installations and require approval before unlink or uninstall

Why

The native-review README rewrite accidentally removed the entry point to agent-guide.md even though the guide itself remained in the repository.

Validation

• verified the raw guide URL on GitHub
• checked plugin commands against the installed Herdr CLI
• npm run version:check
• npm test (41 Node tests and 9 Bun UI tests)
• npm run check
• git diff --check