Rewrite docs/opencode-setup.md so it no longer instructs users to copy
the repo-scoped root AGENTS.md. Instead, document how to install skills
into .opencode/skills/ (project), ~/.config/opencode/skills/ (global),
and cross-compatible Claude/agent paths. Provide a project-local
AGENTS.md template and keep the intent mapping and lifecycle guidance.
Update the README summary to match the new installation model.
Refs #99
Refs #119