chore: extract website to kerrshift/agentdiff-website

[lostmartian]

What

Removes website/ (109 files) from this repository. The site now lives in kerrshift/agentdiff-website — full git history preserved via subtree split — with its own CI/CD: GitHub Actions builds and deploys agentdiff.app to Cloudflare Pages on every merge there.

Why

The product repo mixed Python and TypeScript, which complicated PR rules and CI. This repo is now product-only Python; the site repo owns its site pipeline end to end. First deploy from the new repo is live and verified (agentdiff.app 200, served by the new pipeline).

How

• website/ subtree-split into the new repo (history intact), then removed here.
• Makefile: website-* targets dropped; ci = lint + test + build.
• AGENTS.md / CONTRIBUTING.md: product-website PR-split rules replaced by a pointer to the site repo; worker/ (token service) explicitly stays here as product infrastructure.
• README dev section points to the site repo. CHANGELOG [Unreleased] entry added.

Testing

• make lint → All checks passed
• uv run python -m pytest → 435 passed
• agentdiff.app live-verified after the first deploy from the new repo

Checklist

• Exactly one logical change in this PR
• make lint passes
• uv run python -m pytest is green
• Docs updated if user-facing
• CHANGELOG.md entry added under [Unreleased] (if feat/fix)
• Conventional title: type(scope): summary

Roadmap link: WS-H distribution / repo hygiene.