fix(plugin): drop hooks field from plugin.json (auto-loaded)

Claude Code auto-loads hooks/hooks.json. Declaring it explicitly in
manifest.hooks causes: 'Duplicate hooks file detected ... already-loaded'.
Same pattern as agents: manifest should only reference *additional* hook
files beyond the default.

plugins/.claude-plugin/plugin.json

@@ -24,6 +24,5 @@
     "extraction",
     "mcp"
   ],
-  "skills": "./skills",
-  "hooks": "./hooks/hooks.json"
+  "skills": "./skills"
 }