[Feature flag] Rollout of mfe_enabled

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Summary

Rollout tracking for the mfe_enabled feature flag — the instance-wide gate (together with the static gitlab.yml switch) for the MFE verified-delivery platform.

• Epic: gitlab-org#22777
• Phase 1 issue: #605798
• Introduced by: !245043 (closed)
• Actor type: :instance

Rollout plan

1. Keep disabled everywhere while the Phase 1 stack merges — with the flag off (or the gitlab.yml mfe section absent) the platform is fully inert.
2. Enable in dev/staging environments once a proof-of-concept app is pinned, to exercise bake + same-origin serving.
3. Production enablement only together with the first real pinned application and its per-surface flag.

Removal criteria

The flag guards platform infrastructure rather than a single surface; it graduates to default_enabled: true (keeping the static gitlab.yml kill switch as the operator control) once Phase 2 of the epic (independent release cycle) is operating in production.