feat: add `aeorank-cli doctor` — diagnose AI crawler access in 30s

[vinpatel]

The #1 invisible-to-ChatGPT failure mode is a site that blocks AI crawlers in `robots.txt` or via firewall/WAF rules (Cloudflare Bot Fight Mode, AWS WAF AI-bot blocklists). Users don't realize they're blocked until they run the full scanner.

A `doctor` command would diagnose this in 30 seconds without a full scan.

Scope

`aeorank-cli doctor ` should:

1. Fetch `robots.txt` and list every AI agent (GPTBot, ClaudeBot, PerplexityBot, Google-Extended, CCBot, Bytespider, Amazonbot, …) as allowed / blocked / unspecified.
2. Make a HEAD request as each agent's UA and report the status code.
3. Flag Cloudflare / AWS WAF signals in response headers (`cf-mitigated`, `x-amzn-waf-action`).
4. Check for common firewall blocks on `/llms.txt` specifically (many WAFs serve 403 for unknown paths with .txt extensions).
5. Print a clean table and a one-line verdict: "✅ All major AI crawlers can reach your site" or "❌ 4 of 9 AI crawlers blocked — see details".

Acceptance

• New command in `packages/cli/src/commands/doctor.ts`
• Covers the AI agents in the existing `packages/core` constants
• No false positives against aeorank.dev, vercel.com, stripe.com
• Tests with mocked fetch
• Docs page

Meatier than a good-first-issue but still scoped — ideal for someone who wants to ship a distinct feature.