feat(gateway): add MCP server endpoint with streamable HTTP transport (#52)

- Install @modelcontextprotocol/sdk and zod in apps/gateway
- Create McpModule with McpService and controller at /mcp endpoint
- Implement stateful streamable HTTP transport (MCP spec 2025-03-26)
- Expose 14 tools: brain (projects/tasks/missions), memory, coord
- Require valid BetterAuth session for all MCP connections
- Per-session McpServer + StreamableHTTPServerTransport instances
- Follows same Fastify onRequest hook pattern as auth handler
- Add mcp.dto.ts with McpToolDescriptor and McpServerInfo interfaces

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

apps/gateway/package.json

@@ -15,6 +15,7 @@
     "@fastify/helmet": "^13.0.2",
     "@mariozechner/pi-ai": "~0.57.1",
     "@mariozechner/pi-coding-agent": "~0.57.1",
+    "@modelcontextprotocol/sdk": "^1.27.1",
     "@mosaic/auth": "workspace:^",
     "@mosaic/brain": "workspace:^",
     "@mosaic/coord": "workspace:^",
@@ -47,7 +48,8 @@
     "reflect-metadata": "^0.2.0",
     "rxjs": "^7.8.0",
     "socket.io": "^4.8.0",
-    "uuid": "^11.0.0"
+    "uuid": "^11.0.0",
+    "zod": "^4.3.6"
   },
   "devDependencies": {
     "@types/node": "^22.0.0",