Add landing analytics, waitlist, and favicon

[KenTaniguchi-R]

Summary

• add privacy-minimized PostHog pageview and CTA/conversion tracking
• add a native Formspark waitlist with accessible success/error states and honeypot protection
• add a scalable AgentCall favicon and metadata
• gate Pages deployment on landing-page contract tests
• record the Exa-backed free-tool evaluation

Privacy

• PostHog runs in cookieless mode with autocapture and session recording disabled
• form email values are never sent to analytics
• waitlist_joined fires only after Formspark confirms success

Verification

• pnpm landing:check
• pnpm -r build
• pnpm -r typecheck
• pnpm -r test (1,327 passed, 4 skipped)

Closes #256