docs: add llms-full.txt for AI tools that want to understand agr in one file

The existing llms.txt was only 67 lines — a bare summary. For a project
that's a package manager for AI agents, comprehensive AI discoverability
matters. The new llms-full.txt combines all documentation (commands, config,
SDK, tools, skill creation, troubleshooting) into one structured text file
that any AI model can consume in a single context window.

Also updates llms.txt to link to the full version per the llms.txt standard.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people