feat(go-common): add structured slog logger package

[PramithaMJ]

Introduces packages/go-common/logger — a zero-dependency, context-aware structured logger built on the stdlib log/slog (Go 1.21+):

• New / NewFromEnv: construct a JSON (prod) or text (dev) logger driven by LOG_LEVEL and LOG_FORMAT env vars; installs it as the process-wide default
• WithContext / FromContext: store and retrieve a per-request logger; FromContext auto-enriches every record with tenant_id (from tenant.Middleware) and trace_id so handler code never has to thread these fields manually
• WithTraceID / TraceIDFromContext: propagate trace IDs through context for distributed tracing support
• HTTPMiddleware: request-logging middleware that captures method, path, status, latency, remote_addr, tenant_id and trace_id in one structured record; wraps ResponseWriter to record status without breaking http.Flusher / http.Hijacker via Unwrap()
• 13 table-driven tests covering level filtering, context enrichment, tenant isolation, trace fallback (X-Request-ID), and non-2xx capture