Question about "Direct plugin installs (repos, URLs, local paths) are deprecated. "

Recently (as of version 1.0.35-0 or so), I started getting this warning when running `copilot plugin install /path/to/plugin/on/my/machine`:

```
⚠️ Warning: Direct plugin installs (repos, URLs, local paths) are deprecated. Only plugin@marketplace installs will be supported in a future release.
```

Can anyone shed some light on why this change is being made? Also, will this affect the `--plugin-dir` CLI option too, meaning all custom plugins will have to come from marketplace, or will there always be some local source for custom agents, hooks, etc?

Start with the `copilot plugin install /path/to/plugin/on/my/machine` command and the `--plugin-dir` CLI option mentioned in the report. Check the plugin-installation and CLI documentation or behavior to determine whether local sources remain supported and document the rationale and future scope of the deprecation. Done when both questions have an authoritative answer.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.