feat: one skill, one command, always on, one-line installer (0.6.0)

all-the-medicine is now the only skill and the only command on every
platform. The other 30 public skills are references inside it, so menus
show one entry instead of 31 (verified live: /ai-psychiatry:all-the-medicine).
Always on through a SessionStart hook (exec form), a Codex AGENTS.md block
and a Cursor alwaysApply rule, all printed from one contract file.
install.sh / install.ps1 install Claude Code, Codex and Cursor in one line.
SKILL.md fits Codex's 8,000-byte limit; builders, validator and tests share
one public-skill path rule and fail on a second plugin skill.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
