Add Flipt + OpenFeature integration for runtime feature flags

[haasonsaas]

Context

Feature flags are currently file-based only (featureflags/file_store.go). Services can't toggle features without a deploy. This blocks:

• Kill switches for new features during incidents
• Gradual rollouts to specific orgs/teams
• A/B testing (relevant for evalops/prompts#28)
• Runtime configuration changes

Why Flipt + OpenFeature

• Flipt: Open-source, written in Go, self-hosted, supports Git-based flag definitions (GitOps-native). Uses Postgres or embedded SQLite — no additional infrastructure for small deployments. Native gRPC and HTTP APIs.
• OpenFeature: CNCF vendor-neutral SDK. Abstracts the flag backend so we can swap providers later without changing application code.

Requirements

service-runtime integration

• Add github.com/open-feature/go-sdk dependency
• Add github.com/open-feature/go-sdk-contrib/providers/flipt provider
• Create featureflags/openfeature.go that initializes the Flipt provider from env vars (FLIPT_URL)
• Maintain backward compatibility with file-based store (fallback when FLIPT_URL is unset)
• Add ConnectRPC interceptor that extracts user/org context from request metadata and populates OpenFeature evaluation context automatically

Flipt deployment

• Add Flipt deployment to evalops/deploy (single replica, Postgres-backed, using existing cluster Postgres)
• Configure Git-based flag definitions: flags defined in a flags/ directory in the deploy repo, synced to Flipt via GitOps
• Add Flipt ServiceMonitor for Prometheus scraping
• Add basic Grafana dashboard for flag evaluation metrics

Migration path

• Migrate existing file-based flags to Flipt definitions
• Add FLIPT_URL to service environment configs
• Document flag lifecycle: creation, ownership, expected removal date, stale flag policy (>90 days → review)

Usage pattern

client := openfeature.NewClient("meter")
enabled, _ := client.BooleanValue(ctx, "budget-alerts-v2", false,
    openfeature.NewEvaluationContext("", map[string]interface{}{
        "targetingKey": orgID,
        "tier":         orgTier,
    }),
)