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>

.env.example

@@ -0,0 +1,12 @@
+# Database
+DATABASE_URL=postgresql://mosaic:mosaic@localhost:5432/mosaic
+
+# Valkey (Redis-compatible)
+VALKEY_URL=redis://localhost:6379
+
+# OpenTelemetry
+OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318
+OTEL_SERVICE_NAME=mosaic-gateway
+
+# Gateway
+GATEWAY_PORT=4000