Migrate plugin commands into skills on install (#33411)

## What changed

- Convert supported plugin command Markdown into generated skills when a plugin is installed.
- Read command sources from the manifest's `commands` field, falling back to the plugin's `commands/` directory.
- Load generated command skills alongside native plugin skills while preferring native skills with the same name.
- Skip commands with unsupported templates, missing descriptions, name collisions, or generated skills larger than 4 KB, and keep migration failures from blocking plugin installation.

## Testing

- Cover default and manifest-configured command paths, stale generated skill cleanup, size limits, native-skill precedence, invalid manifest fields, and non-fatal migration errors.

GitOrigin-RevId: 67d910048544b160b191066569bca69b2b951ebc

codex-rs/Cargo.lock

@@ -2821,6 +2821,7 @@ dependencies = [
  "semver",
  "serde",
  "serde_json",
+ "serde_yaml",
  "tar",
  "tempfile",
  "thiserror 2.0.18",