docs: add AGENTS.md + cleanup

[kehoecj]

Adds AGENTS.md — a cross-platform project context file that AI coding agents (Claude Code, Codex, Kiro, etc.) read automatically when working in this repo.

What's in it

• Project architecture and data flow overview
• Full local CI pipeline (replicates what GitHub Actions runs)
• Step-by-step recipes for adding a new validator or reporter
• PR requirements (CHANGELOG, conventional commits, coverage threshold)
• golangci-lint pitfalls that commonly trip up contributors
• Dependency policy, security constraints, testing patterns
• Documentation site build instructions
• MegaLinter and OpenSSF Scorecard guidance

Also included

• README.md: Fix inconsistent install section headings (bold text vs h3 inside <details>)
• CONTRIBUTING.md: Add pointer to AGENTS.md as a quick-start reference
• website/.npmrc: Pin npm registry to public npmjs.org to prevent internal registry URLs leaking into package-lock.json

Why

Contributors using AI tools currently get no project-specific guidance — the agent has to reverse-engineer patterns by reading multiple files. This gives it the full picture upfront, which means fewer broken PRs and less reviewer back-and-forth on things like missing CHANGELOG entries, wrong import ordering, or forgetting to register a file type in all three places.