feat: add PostToolUse hook to auto-validate marketplace.json on edit

When marketplace-dev is installed, any Write/Edit to a marketplace.json
automatically runs `claude plugin validate` and reports pass/fail.
Users get instant feedback without remembering to validate manually.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -960,7 +960,20 @@
       ],
       "skills": [
         "./marketplace-dev"
-      ]
+      ],
+      "hooks": {
+        "PostToolUse": [
+          {
+            "matcher": "Write|Edit",
+            "hooks": [
+              {
+                "type": "command",
+                "command": "${CLAUDE_PLUGIN_ROOT}/scripts/post_edit_validate.sh"
+              }
+            ]
+          }
+        ]
+      }
     }
   ]
 }