feat: tool registration — brain tools for agent sessions (P2-004)

Register brain data layer as LLM-callable tools in Pi SDK agent
sessions. Agents can now query/create/update projects, tasks,
missions, and conversations through tool calls.

- 7 brain tools: list/get projects, list/create/update tasks,
  list missions, list conversations
- Tools registered via customTools in createAgentSession()
- Uses TypeBox schemas for parameter validation

Closes #22

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/gateway/package.json

@@ -30,6 +30,7 @@
     "@opentelemetry/sdk-metrics": "^2.6.0",
     "@opentelemetry/sdk-node": "^0.213.0",
     "@opentelemetry/semantic-conventions": "^1.40.0",
+    "@sinclair/typebox": "^0.34.48",
     "better-auth": "^1.5.5",
     "fastify": "^5.0.0",
     "reflect-metadata": "^0.2.0",