feat(#329): Add usage budget management and cost governance

Implement BudgetService for tracking and enforcing agent usage limits:
- Daily token limit tracking (default 10M tokens)
- Per-agent token limit enforcement (default 2M tokens)
- Maximum concurrent agent cap (default 10)
- Task duration limits (default 120 minutes)
- Hard/soft limit enforcement modes
- Real-time usage summaries with budget status
  (within_budget/approaching_limit/at_limit/exceeded)
- Per-agent usage breakdown with percentage calculations

Includes BudgetModule for NestJS DI and 23 unit tests.

Fixes #329

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/orchestrator/src/budget/budget.module.ts

@@ -0,0 +1,10 @@
+import { Module } from "@nestjs/common";
+import { ConfigModule } from "@nestjs/config";
+import { BudgetService } from "./budget.service";
+
+@Module({
+  imports: [ConfigModule],
+  providers: [BudgetService],
+  exports: [BudgetService],
+})
+export class BudgetModule {}