Add high-intent CTA section and keyboard skip link

[mouse-value-add]

Summary

This PR adds a focused conversion and accessibility improvement to the homepage in one deploy-safe scope:

• Adds a new primary CTA section above Connect with a clear collaboration value prop
• Adds two high-intent actions: Email Brian and Message on LinkedIn
• Adds GA click events for both CTA buttons (event_category: CTA)
• Adds a skip-to-content link for keyboard/screen-reader users
• Wraps page content in a main landmark (#main-content) for better semantics

Why this helps

The page had strong narrative and credibility, but no explicit conversion step. This introduces a clear next action for qualified visitors while also improving keyboard accessibility with minimal risk.

Validation

• git diff --check (pass)
• Local static serve check: python3 -m http.server + curl -I /index.html returned HTTP/1.0 200 OK

Note: xmllint --html reports existing HTML5/legacy entity warnings in the current page that predate this PR.