fix: add llms.txt

[murilopmachado]

Summary

• Adds web/llms.txt, the AI-crawler-readable counterpart to web/robots.txt, so LLMs/AI tools can quickly understand what structlint is and does.
• Content is grounded in the README (checks, run modes, output formats) — no invented stats or claims.
• Does not touch existing robots.txt or the SEO meta tags in web/index.html.

Test plan

• File sits at web/llms.txt, next to web/robots.txt
• Content matches the format/tone of the portfolio's existing llms.txt files

[github-actions]

✅ PR title follows the required format

Current title: fix: add llms.txt