feat(platform): Phase 3 — AI Budget & Cost Governance
- Scope expansion: BudgetScopeTypeSchema now includes 'org' + 'workspace'
- Cost dashboard: GET /ai-budgets/costs with groupBy (model/agent/day/scope)
- Aggregates totalCostUsd, totalTokens, entryCount, breakdown
- Budget rollover: POST /ai-budgets/policies/:id/rollover
- Computes previous period remaining, creates rollover doc, adjusts budget
- GET /ai-budgets/policies/:id/rollovers for history
- Enforcement check: POST /ai-budgets/check (pre-flight, no spend recorded)
- Model allowlist + threshold evaluation, returns verdict + reasons
- New types: CostDashboardQuerySchema, BudgetRolloverSchema
- New repo functions: listAllSpendEntries, createRollover, listRollovers
- New Cosmos container: ai_budget_rollovers
- 1,316 tests passing (9 new)
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