Add ADR 015: Non-isolation is a permanent Organization state

Summary

• Adds ADR 016: a non-isolated Organization is a valid, permanent state, not a phase every Organization is expected to leave. Isolation is something an Organization's owner opts into, not a universal destination.
• Updates _index.md's "Organization Isolation" section and decisions list to reflect this.

This is deliberately small in scope so it can merge independently. A separate, upcoming MR builds on this to define Organization-scoped request context (Org/User/Nil context) and depends on non-isolation being permanent, not transitional.

Test plan

• Confirm ADR 016 accurately reflects the intended model: isolation is opt-in, not a rollout every Organization completes.
• Confirm the _index.md changes don't overstate anything ADR 016 doesn't actually decide (for example, it does not rewrite isolation.md itself).