docs: README overhaul — hero, demos, feature grid, agent section

[KalebCole]

Overview

The Partiful CLI README needs a major glow-up. Currently functional but doesn't showcase the tool well.

Proposed Structure

1. Hero section — Logo/banner, one-liner, badges (version, license, npm downloads)
2. Quick start — 3 commands: install → auth → first event
3. Feature grid — Visual showcase of capabilities (events, guests, blasts, contacts, posters, templates, bulk, watch)
4. Terminal demos — GIF recordings of key flows:
   • Create an event with a poster
   • Search contacts and invite by name
   • Send a text blast
   • Export guest list
5. Agent integration — Section showing how AI agents use the CLI (OpenClaw, Claude Code)
6. Architecture overview — Brief diagram of how it interfaces with Partiful
7. Full command reference — Generated from partiful schema

Inspiration

• gh CLI README — clean, badge-heavy, feature sections
• Railway CLI — good quick start flow
• Vercel CLI — clean hero + feature grid