- clarify app mentions are in user messages
- clarify what it means for tools to be provided via `codex_apps` MCP
- add plugin descriptions (with basic sanitization) to top-level `##
Plugins` section alongside the corresponding plugin names
- explain that skills from plugins are prefixed with `plugin_name:` in
top-level `##Plugins` section
changes to more logically organize `Apps`, `Skills`, and `Plugins`
instructions will be in a separate PR, as that shuffles dev + user
instructions in ways that change tests broadly.
### Tests
confirmed in local rollout, some new tests.
