feat(kiosk): plugin.yaml manifest and summer plugin:manifest

- new module kiosk: Manifest, Marshal, Parse, Derive, Generate, Write, Check, StaleError
- plugin:manifest [dir] with --check, --pretend, --name, --description
- docs: kiosk README, root table row, plugins/marketplace.md, scaffolding row

docs/console/scaffolding.md

@@ -103,6 +103,13 @@ summer plugin:add plugins/acme/blog
 
 `summer plugin:add` points the application's `replace` and `go.work` at the new directory; the module path and the `summer.yaml` entry stay as they were. If the plugin's own `go.mod` has a relative `replace` for the framework (or another local module), add one `../` to it, for example `../../../summercms` becomes `../../../../summercms`. Then run `summer build`.
 
+## Plugin commands
+
+| Command | Purpose |
+|---------|---------|
+| `summer plugin:add` | Registers a local plugin directory: `summer.yaml`, a `require` and a local `replace` in `go.mod`, and `go.work`. |
+| `summer plugin:manifest` | Writes the plugin's `plugin.yaml` from its sources (`plugin.go`, `go.mod`, the README). `--pretend` prints it, `--check` exits 1 when the committed file is missing or stale, `--name` and `--description` override the README text. See [Publishing plugins](../plugins/marketplace.md). |
+
 ## Generating plugin parts
 
 The other `make:` commands add one artifact to an existing plugin. Each takes the plugin ID and an exported Go name: