## Summary
Allow plugin manifests to declare `skills` as either a single path
string or an array of path strings in the core plugin loader.
## Why
Some plugin packages need to expose skills from more than one directory.
Before this change, `plugin.json` only accepted a single string for
`skills`, so manifests like this were ignored as an invalid `skills`
shape:
```json
{
"skills": ["./skills/abc", "./skills/edk"]
}
```
This keeps the existing single-string form working while adding support
for the list form. The final scope is intentionally limited to the core
plugin manifest/load path for `skills`; `apps`, file-backed
`mcpServers`, and the bundled plugin-creator assets are unchanged in
this PR.
## What changed
- Parse `skills` as either a string or an array of strings in
`plugin.json`.
- Store resolved skill paths as a list in `PluginManifestPaths`.
- Load manifest-declared skill roots in addition to the default
`./skills` root.
- Deduplicate exact duplicate skill roots before loading.
- Rely on existing skill-loader dedupe by canonical `SKILL.md` path for
overlapping roots such as `./skills` plus `./skills/abc`.
- Update plugin manifest tests to cover:
- single string `skills`
- list of string `skills`
- duplicate skill roots
- `./skills` as a manifest path
- explicit child roots like `./skills/abc` and `./skills/edk`
- overlapping-root dedupe
## Validation
- `just test -p codex-plugin`
- `just test -p codex-core-plugins`
- `just test -p codex-mcp-extension`
- `git diff --check`
charlesgong-openai
e12dd73b7d