docs(skills): spell Plugin.type as PluginType in objectstack-platform (#14981)

The skill's copy of the Plugin interface still spelled type as an open
string with the value set in a trailing comment. #13925 narrowed the
published field to the closed PluginType (exported from
@objectstack/core), so the skill was teaching a wider accept set than
the compiler enforces. Same-length spelling fix at both sites, with
the enumeration comment replaced by a pointer to PluginType.


Claude-Session: https://claude.ai/code/session_019RfFHiRCSs3JXLK4cwcfox

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-steve

skills/objectstack-platform/SKILL.md

@@ -720,7 +720,7 @@ import type { Plugin, PluginContext } from '@objectstack/core';
 export interface Plugin {
   name: string;               // Unique identifier (reverse domain recommended)
   version?: string;           // Semantic version
-  type?: string;              // standard|ui|driver|server|app|theme|agent|objectql
+  type?: PluginType;          // closed set exported by @objectstack/core
   dependencies?: string[];    // Plugins that must init before this one
 
   // Phase 1: Register services

skills/objectstack-platform/rules/plugin-lifecycle.md

@@ -45,7 +45,7 @@ export interface Plugin {
   version?: string;
 
   /** Plugin type */
-  type?: string;  // standard|ui|driver|server|app|theme|agent|objectql
+  type?: PluginType;  // closed set exported by @objectstack/core
 
   /** Plugins that must init before this one */
   dependencies?: string[];