Org Read-Only Mode: feature flag gating for cohort-by-cohort rollout

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Part of &20404 (Implement Organizations Read-Only Mode). Design: ADR 010.

• Environment-scoped and Organization-scoped feature flag gating so rollout can proceed cohort-by-cohort
• On GitLab.com: enable first for internal/test Organizations, then expand alongside existing Organizations rollout cohorts
• Default-off on Self-Managed and Dedicated (instance-wide Maintenance Mode remains the right tool there, see ADR 007)