[Story]: Migrate flat accounts/projects onto the organization hierarchy

## Summary
Migrate today's flat accounts/projects onto the new organization hierarchy as a default single-level tree, with zero disruption to live budget enforcement.

## Intent / Source of truth
The hierarchy is only useful if existing customers can adopt it without a breaking migration; live budget enforcement (ai-helm ADR-0021/0035) must keep working throughout. Part of [Epic] Organization hierarchy and delegated administration.

## Scope
- [ ] Migration script mapping every existing account/project to a hierarchy node
- [ ] Shadow-mode verification (hierarchy resolves identical policy to today's flat model before cutover)
- [ ] Rollback plan
- [ ] Post-migration budget-enforcement regression suite run

## Out of scope
- Customer-initiated re-organization into a deeper hierarchy (that's a follow-on, self-service action)

## Verification
Shadow-mode diff report shows zero policy-resolution deltas versus the current flat model for 100% of existing accounts; budget-enforcement regression suite green post-migration.

## Risk assessment
This is the highest-risk story in the epic — a migration bug could silently change a live customer's budget or model access. Requires shadow-mode verification before any cutover, per working-preferences (verify LIVE before "done").

## AI Usage Declaration
Drafted with AI assistance during the 2026-08-13 cross-repo backlog consolidation and enterprise-readiness research. A human owns intent, verification and consequences.