docs: address plugin guide review feedback

Co-Authored-By: Paperclip <noreply@paperclip.ing>

doc/plugins/PLUGIN_AUTHORING_GUIDE.md

@@ -248,7 +248,6 @@ const manifest: PaperclipPluginManifestV1 = {
     {
       skillKey: "weekly-brief-skills",
       displayName: "Weekly Briefer",
-      slug: "weekly-brief-skills",
       description: "Reusable skill for the managed research workflow.",
     },
   ],

doc/plugins/PLUGIN_SPEC.md

@@ -345,6 +345,7 @@ export interface PaperclipPluginManifestV1 {
   routines?: PluginManagedRoutineDeclaration[];
   skills?: PluginManagedSkillDeclaration[];
   localFolders?: PluginLocalFolderDeclaration[];
+  /** Legacy top-level launcher declarations. Prefer `ui.launchers` for new manifests. */
   launchers?: PluginLauncherDeclaration[];
   ui?: {
     launchers?: PluginLauncherDeclaration[];