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>

docker-compose.yml

@@ -0,0 +1,51 @@
+services:
+  postgres:
+    image: pgvector/pgvector:pg17
+    ports:
+      - '5432:5432'
+    environment:
+      POSTGRES_USER: mosaic
+      POSTGRES_PASSWORD: mosaic
+      POSTGRES_DB: mosaic
+    volumes:
+      - pg_data:/var/lib/postgresql/data
+    healthcheck:
+      test: ['CMD-SHELL', 'pg_isready -U mosaic']
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  valkey:
+    image: valkey/valkey:8-alpine
+    ports:
+      - '6379:6379'
+    volumes:
+      - valkey_data:/data
+    healthcheck:
+      test: ['CMD', 'valkey-cli', 'ping']
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  otel-collector:
+    image: otel/opentelemetry-collector-contrib:0.100.0
+    ports:
+      - '4317:4317' # OTLP gRPC
+      - '4318:4318' # OTLP HTTP
+    volumes:
+      - ./infra/otel-collector.yml:/etc/otelcol-contrib/config.yaml:ro
+    depends_on:
+      jaeger:
+        condition: service_started
+
+  jaeger:
+    image: jaegertracing/jaeger:2
+    ports:
+      - '16686:16686' # Jaeger UI
+      - '4319:4317' # Jaeger OTLP gRPC (internal, collector forwards here)
+    environment:
+      COLLECTOR_OTLP_ENABLED: 'true'
+
+volumes:
+  pg_data:
+  valkey_data: