feat: Spring Boot starter (auto-config, actuator, Micrometer)

[hermanngeorge15]

Summary

Implement Spring Boot auto-configuration for budget-breaker in the budget-breaker-spring-boot-starter module.

Scope

• @ConfigurationProperties for budget.breaker.* namespace
• Auto-create BudgetCircuitBreaker bean with properties-based config
• /actuator/budget endpoint showing active agent budgets and reports
• Micrometer integration:
  • budget.tokens.total counter (prompt + completion)
  • budget.cost.estimated gauge
  • budget.soft_limit.breaches counter
  • budget.hard_limit.breaches counter
  • budget.duration timer

Acceptance Criteria

• @EnableBudgetBreaker or auto-config via spring.factories/AutoConfiguration.imports
• Properties-based budget configuration
• Actuator endpoint returns JSON with all active reports
• Micrometer metrics registered and updated on each trackCall()
• Integration test with @SpringBootTest