fix: declare prompt_rules tool in contracts.tools

Same class of bug as Meridian (zhi/Meridian#2), HarborForge.OpenclawPlugin
(zhi/HarborForge.OpenclawPlugin#6), and PaddedCell (already fixed in
787d88c). OpenClaw's plugin host requires that any tool registered via
`api.registerTool()` is also declared in `contracts.tools` in the
plugin manifest, or the tool is silently dropped from the agent's
available tool list. plugin doctor was warning:

  prism-facet: plugin must declare contracts.tools before registering
  agent tools

PrismFacet registers exactly one tool, `prompt_rules` (in
tools/prompt-rules.js). Declaring it in the manifest. Verified in sim
that with this change the warning disappears and the tool becomes
visible to agents.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/openclaw.plugin.json

@@ -17,5 +17,10 @@
         "description": "Path to rules.json (default: {pluginDir}/rules.json)"
       }
     }
+  },
+  "contracts": {
+    "tools": [
+      "prompt_rules"
+    ]
   }
-}
+}