feat: add token tracking and budget warnings

[nigel-dev]

Description

Deferred from v1.5 release. Implement token usage tracking and budget warning system.

Background

In v1.5, we implemented the permission policy engine (#69) without token/cost tracking. This issue covers the remaining cost-related features that were explicitly deferred.

Requirements

• Track token usage per job via SSE message events
• Configurable budget limits per job and per plan
• Warning notifications when approaching budget thresholds
• Budget enforcement: pause/stop jobs exceeding limits
• Cost estimation based on model pricing (configurable)
• All tracking OFF by default, configurable via MCConfig

Related

• Deferred from: feat: Job governance and cost control — permission policies, token tracking, budget limits #69
• Builds on: feat: Migrate to serve-based orchestration with SDK monitoring and enhanced mc_attach #65 (serve-mode SSE events provide token metadata)