feat: foundation — Docker Compose, OTEL, shared types (P0-002, P0-005, P0-006)

Docker Compose (P0-005):
- PG 17 + pgvector, Valkey 8, OTEL Collector, Jaeger
- OTEL Collector config with trace/metric/log pipelines
- .env.example with all connection strings

OTEL Foundation (P0-006):
- @opentelemetry/sdk-node with auto-instrumentations
- tracing.ts loaded before NestJS bootstrap
- OTLP HTTP exporter → collector → Jaeger
- Disabled fs/dns instrumentations (noisy)

Shared Types (P0-002):
- ChatMessageDto with class-validator (IsUUID, IsNotEmpty, MaxLength)
- ChatResponseDto for REST responses
- Socket.IO typed event maps (ServerToClientEvents, ClientToServerEvents)
- AgentSessionHandle opaque type
- All event payload interfaces shared across gateway/cli/discord

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

apps/gateway/package.json

@@ -12,12 +12,19 @@
     "test": "vitest run"
   },
   "dependencies": {
+    "@mariozechner/pi-coding-agent": "~0.57.1",
     "@nestjs/common": "^11.0.0",
     "@nestjs/core": "^11.0.0",
     "@nestjs/platform-fastify": "^11.0.0",
     "@nestjs/platform-socket.io": "^11.0.0",
     "@nestjs/websockets": "^11.0.0",
-    "@mariozechner/pi-coding-agent": "~0.57.1",
+    "@opentelemetry/auto-instrumentations-node": "^0.71.0",
+    "@opentelemetry/exporter-metrics-otlp-http": "^0.213.0",
+    "@opentelemetry/exporter-trace-otlp-http": "^0.213.0",
+    "@opentelemetry/resources": "^2.6.0",
+    "@opentelemetry/sdk-metrics": "^2.6.0",
+    "@opentelemetry/sdk-node": "^0.213.0",
+    "@opentelemetry/semantic-conventions": "^1.40.0",
     "fastify": "^5.0.0",
     "reflect-metadata": "^0.2.0",
     "rxjs": "^7.8.0",