Standardize structured logging on slog across all services

[haasonsaas]

Problem

Logging is fragmented across 3 libraries with no standard:

Logger	Services
log.Printf() (unstructured)	identity, keys, memory, meter, audit, registry
log/slog (structured)	llm-gateway, prompts, parker, service-runtime
zerolog (structured)	gate

Six services use the standard library's log package — unstructured text to stderr. In production this means:

• Can't filter by log level
• Can't search by structured field (org ID, request ID, user ID)
• Can't correlate logs across requests
• Log aggregation tools (Loki, CloudWatch, etc.) can't parse them

Proposal

1. Standardize on log/slog (stdlib, no extra dependency, already used by 4 services + service-runtime)
2. Provide a NewLogger() factory in service-runtime that configures:
   • JSON handler in production (ENVIRONMENT=production)
   • Text handler in development
   • Default attributes: service name, version, environment
3. Require request-scoped logging via middleware that injects request ID, org ID, and actor into the slog context
4. Migrate gate from zerolog to slog (or accept the divergence — gate is already structured)

Migration path

• service-runtime already has slog primitives in observability/logging.go
• Replace log.Printf() → slog.Info() / slog.Error() with structured fields
• Each service migration is a small PR (mostly find-and-replace)

Services to migrate

• identity
• keys
• memory
• meter
• audit
• registry
• gate (optional — already structured via zerolog)