The Codex CLI requires .codex-plugin/plugin.json as the plugin manifest
entry point. Without it, 'codex plugin add' fails with 'missing plugin.json'
even though the marketplace listing works.
This adds .codex-plugin/plugin.json to all 14 plugin directories, with paths
relative to the plugin root per the Codex docs. Also updates the agents
marketplace to use dotnet-aspnetcore (per #711 rename) and adds missing
dotnet-blazor and dotnet11 entries.
Fixes#578Fixes#724
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>