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

modules/kiosk/testdata/literal/README.md

@@ -0,0 +1,7 @@
+# Blog
+
+Posts and categories for an `acme` application, with a feed.
+
+## Overview
+
+More text that is not part of the manifest.

modules/kiosk/testdata/literal/go.mod

@@ -0,0 +1,5 @@
+module example.test/sm-acme-plugin
+
+go 1.27.0
+
+require git.golem15.com/golem15/summercms v0.5.3

modules/kiosk/testdata/literal/plugin.go

@@ -0,0 +1,12 @@
+// Package blog is a test fixture: a plugin whose ID() and Requires()
+// return literals. kiosk reads it with go/parser; it is never built.
+package blog
+
+// Plugin is the fixture plugin.
+type Plugin struct{}
+
+// ID returns the plugin ID.
+func (*Plugin) ID() string { return "acme.blog" }
+
+// Requires returns no plugins.
+func (*Plugin) Requires() []string { return nil }