feat(v0.4.0): OpenCode + Pi primitive install — commands, flat skills, command-type hooks

[espetro]

Summary

Make installed plugins fully functional on OpenCode and Pi:

1. Workstream A — Flatten skill linking (fixes Pi skill collisions)
2. Workstream B — OpenCode adapter emits commands + agents + command-type hook handlers
3. Workstream C — Store multi-artifact linking for OpenCode (command/.md, agent/.md, plugins/*.ts)

Plan

See .agents/plans/2026-06-26-opencode-pi-primitive-install.md

Effort: L

Classification: feature