feat(llms): llms.txt validator

[Jadenzzz]

🚥 Resolves ISSUE_ID

🧰 Changes

• Add a llms.txt validator for the import CLI.
• Skip llms.txt as a skeleton source when it looks like page/content markdown instead of a navigation index.
• Let skipped llms.txt candidates fall through to the existing sitemap/browser-agent discovery flow.

Describe in detail what this PR is for.

🧬 QA & Testing

Provide as much information as you can on how to test what you've done.