Add llms.txt for AI visibility (GEO)

[jackwener]

Summary

• Adds llms.txt at the repository root — a structured, AI-readable description of OpenCLI following the llms.txt standard
• Helps AI search crawlers (ChatGPT, Perplexity, Claude) accurately understand and cite this project when users ask about browser automation, CLI tools, or AI agent tooling
• Content covers: what OpenCLI does, key capabilities, install steps, supported sites, skills for AI agents, exit codes, and key links
• Cross-references https://opencli.info/llms.txt (already deployed) for network discovery

Why

83% of AI Overview citations come from pages outside Google's top 10 — AI looks for clear structure, not PageRank. A llms.txt gives crawlers an explicit, unambiguous entry point for this repo.

Test plan

• Verify https://raw.githubusercontent.com/jackwener/OpenCLI/main/llms.txt is accessible after merge
• After a few days, search "opencli" or "jackwener opencli" in ChatGPT/Perplexity and check citation accuracy