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

evals/results.json

@@ -1,5 +1,5 @@
 {
-  "generatedAt": "2026-06-18T20:35:19.929Z",
+  "generatedAt": "2026-06-18T20:13:31.726Z",
   "judge": "claude-opus-4-8",
   "models": [
     "claude-sonnet-4-6",
@@ -19,9 +19,9 @@
         "structure": 5,
         "completeness": 5,
         "usefulness": 5,
-        "grounding": 4
+        "grounding": 5
       },
-      "overall": 4.75
+      "overall": 5
     },
     {
       "skill": "rice-prioritisation",
@@ -30,9 +30,9 @@
         "structure": 5,
         "completeness": 5,
         "usefulness": 5,
-        "grounding": 5
+        "grounding": 4
       },
-      "overall": 5
+      "overall": 4.75
     },
     {
       "skill": "prd-template",
@@ -63,9 +63,9 @@
         "structure": 5,
         "completeness": 5,
         "usefulness": 5,
-        "grounding": 5
+        "grounding": 4
       },
-      "overall": 5
+      "overall": 4.75
     },
     {
       "skill": "cs-health-scorecard",
@@ -81,22 +81,22 @@
     {
       "skill": "executive-summary",
       "model": "claude-sonnet-4-6",
-      "scores": {
-        "structure": 5,
-        "completeness": 5,
-        "usefulness": 5,
-        "grounding": 4
-      },
-      "overall": 4.75
-    },
-    {
-      "skill": "executive-summary",
-      "model": "claude-haiku-4-5-20251001",
       "scores": {
         "structure": 5,
         "completeness": 5,
         "usefulness": 4,
-        "grounding": 3
+        "grounding": 4
+      },
+      "overall": 4.5
+    },
+    {
+      "skill": "executive-summary",
+      "model": "claude-haiku-4-5-20251001",
+      "scores": {
+        "structure": 5,
+        "completeness": 4,
+        "usefulness": 4,
+        "grounding": 4
       },
       "overall": 4.25
     },
@@ -116,11 +116,11 @@
       "model": "claude-haiku-4-5-20251001",
       "scores": {
         "structure": 5,
-        "completeness": 4,
+        "completeness": 5,
         "usefulness": 5,
-        "grounding": 3
+        "grounding": 4
       },
-      "overall": 4.25
+      "overall": 4.75
     },
     {
       "skill": "sprint-planning",