feat(skill): add opencode-qa QA skill

Local skill for QAing opencode itself, per case: CLI/terminal, plugin\nhook/action/event via the SSE event stream, TUI under tmux, and session\ninvestigation in the SQLite DB by id/name/text. Ships SKILL.md, 7 domain\nreferences, and 8 tested helper scripts (each with a --self-test).