ag402 setup uses interactive prompts (input(), getpass()) that block
AI agents. ag402 init is non-interactive and produces the same result:
test wallet + $100 USDC.

Changes:
- llms.txt: split Quick Start into AI-agent vs human-guided paths,
  mark ag402 setup as interactive in CLI table, add warning about
  blocking prompts
- buy SKILL.md: add ag402 init as autonomous alternative in Step 2,
  add AI-autonomous quick reference path
- sell SKILL.md: clarify ag402 upgrade requires human input
- All three files now explicitly tell AI agents: "use ag402 init,
  never pipe input to ag402 setup"

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>