## Why
Agent Plugins use a schema-declared root `plugin.json` and can have dotted names or versions that do not fit Codex's directory-safe version format. The packaging and installation paths still assumed the legacy manifest layout and identifier rules.
## What changed
- Recognize valid root Agent Plugin manifests when discovering, packing, and installing plugins, while leaving unrelated root manifests on the legacy path.
- Accept safe dotted plugin names, default missing Agent Plugin versions to `1.0.0`, and derive stable directory-safe versions when necessary without rewriting the portable manifest.
- Skip legacy command migration for Agent Plugins and reject symlinks or other unsupported file types while copying plugin sources.
## Testing
Add coverage for portable bundle round trips, manifest discovery, dotted names, version handling, command preservation, and symlink rejection.
GitOrigin-RevId: 61476c4c4100495842253d8b429c0b896490962d
Move plugin tar.gz packing and unpacking into a shared core-plugins
archive helper so uploaded bundles are decoded through the same tar
handling used for installs. This removes duplicate archive logic,
supports GNU long-name entries on extraction, and keeps size, traversal,
link, and entry-type checks in one place.
