fix(data-platform): correct invalid plugin.json manifest format

- Remove invalid "agents": ["./agents/"] - agent .md files don't need registration
- Inline hooks instead of external reference "hooks/hooks.json"
- Delete orphaned hooks.json file (content now in plugin.json)

This fixes "invalid input" validation errors for hooks and agents fields.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/data-platform/.claude-plugin/plugin.json

@@ -18,8 +18,14 @@
     "etl",
     "dataframe"
   ],
-  "hooks": "hooks/hooks.json",
+  "hooks": {
+    "SessionStart": [
+      {
+        "type": "command",
+        "command": "${CLAUDE_PLUGIN_ROOT}/hooks/startup-check.sh"
+      }
+    ]
+  },
   "commands": ["./commands/"],
-  "agents": ["./agents/"],
   "mcpServers": ["./.mcp.json"]
 }