Restore public README lost in #20, bump to 0.3.1

[teallarson]

Summary

• PR Remove chat, use arcade design tokens in py agent #20 accidentally reverted the external-facing README that Clean up README for public consumers, add CONTRIBUTING.md #19 introduced (likely a merge conflict resolved the wrong way)
• Restores the README from Clean up README for public consumers, add CONTRIBUTING.md #19: Quick start section, Make it yours guide, Key files table, cleaner developer-friendly framing, Node.js >= 22 requirement
• Removes the Development/Scripts/CI/Releasing sections that Clean up README for public consumers, add CONTRIBUTING.md #19 had moved to CONTRIBUTING.md (they'd crept back in)
• Bumps version to 0.3.1

Test plan

• Verify README renders correctly on GitHub
• Confirm CONTRIBUTING.md still has the dev/release docs

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