docs(opencode): real install paths (fixes #235) and the native plugin route

docs/opencode.md claimed that npx skills add -g installs the skill to
~/.config/opencode/skills/; it installs to ~/.agents/skills/, which
OpenCode reads natively alongside ~/.claude/skills/,
~/.config/opencode/skills/ and the project-local .agents/skills/,
.claude/skills/ and .opencode/skills/. The page is rewritten around the
real paths and the plugin route (opencode.json "plugin", commands, the
gate and its limits, session catchup, oh-my-opencode and superpowers
notes kept). The .opencode skill's restore-context snippets and file
location table probe every location, and an OpenCode Notes section
explains that the hooks: frontmatter is a Claude Code convention OpenCode
ignores.

README, MIGRATION.md, docs/installation.md and the English SKILL.md
host tier tables now say what each OpenCode install gets: the native
plugin is Tier 2 (follow-up inject), skill-only installs stay notify
only.

Reported by @luyanfeng in #235.