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/hooks/hooks.json

@@ -1,10 +0,0 @@
-{
-  "hooks": {
-    "SessionStart": [
-      {
-        "type": "command",
-        "command": "${CLAUDE_PLUGIN_ROOT}/hooks/startup-check.sh"
-      }
-    ]
-  }
-}