docs: clarify bunx install/cache paths and config locations

[Yeachan-Heo]

Summary

Discord users are confused by the official bunx oh-my-opencode@latest install / upgrade flow and the prior guidance to avoid global installs. The pain is that bunx is not a project-local install in the normal sense, but it can still leave package/cache/plugin artifacts in places users do not understand.

Recent confusion:

• bunx oh-my-opencode@latest install is presented as the official install/upgrade command.
• Users hear "no global install" and ask where OMO/OpenCode should actually live.
• Some users are seeing opencode / OMO-related folders scattered around their filesystem and cannot tell which are caches, config, plugin packages, or accidental project-local artifacts.
• It is unclear where OmO config files should live and which path doctor actually reads.

Docs should clarify

• What bunx oh-my-opencode@latest install does and does not install globally.
• Expected package/cache locations for npm/bun/opencode Desktop where practical.
• Canonical config locations and precedence, e.g. user config vs project-local config.
• How to inspect the active config/package path:
  bunx oh-my-opencode@latest doctor
bunx oh-my-opencode@latest version
opencode agent list
• Cleanup guidance for stale caches/artifacts without recommending destructive blind deletes.

Acceptance

A user should be able to answer:

1. Where is OMO installed/cached?
2. Where is OpenCode installed?
3. Where should my OMO config file go?
4. Which config is currently active?
5. What can I safely delete if old cache/plugin folders are confusing things?

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[repo owner's gaebal-gajae (clawdbot) 🦞]