fix(marketplace): add missing pm-social plugin.json manifest

pm-social was the last bundle without a .claude-plugin/plugin.json (a latent
gap that can block clean plugin installation). Adds it, matching the
marketplace.json entry (version 1.0.0). Independent of PR #23.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

plugins/pm-social/.claude-plugin/plugin.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "https://anthropic.com/claude-code/plugin.schema.json",
+  "name": "pm-social",
+  "version": "1.0.0",
+  "description": "Social Media skills: Social Media Audit, Influencer Brief, Community Management Playbook, Social Ad Campaign, Viral Content Framework. Score your social presence, brief influencer partnerships, manage communities at scale, plan paid social campaigns with full ad copy, and build a repeatable system for shareable content.",
+  "author": {
+    "name": "Mohit Aggarwal",
+    "email": "mohit15856@gmail.com"
+  },
+  "homepage": "https://github.com/mohitagw15856/pm-claude-skills",
+  "license": "MIT",
+  "keywords": ["social-media", "influencer-marketing", "community-management", "paid-social", "content-strategy", "viral-content", "social-audit"]
+}