feat: add footer waitlist CTA to private beta form

[EthanThatOneKid]

Summary

Adds a footer call-to-action linking to the existing private beta Google Form, per wazoo.dev#5 (Keep Google Form + CTA approach).

• Footer CTA above social icons: "Building agents that need durable memory? Join the private beta." with a Request access button.
• Reuses the existing beta-button style; adds a small footer-cta rule.

Validation

• deno fmt
• git diff --check

[EthanThatOneKid]

Closing in favor of the existing private beta callout admonition on the landing page.