Feature: Cost/Token Circuit Breakers for Autonomous Loops

**Is your feature request related to a problem? Please describe.**
As developers build autonomous loops (e.g., self-healing code agents), there is a significant risk of runaway costs due to infinite loops or hallucinations. Currently, limiting `max_iterations` is a proxy, but it doesn't account for token density or model cost variance.

**Describe the solution you'd like**
A first-class `CircuitBreaker` policy or middleware that can be attached to an Agent or Workflow.
- **Configurable Limits**: Set hard caps on `TotalTokens` or `EstimatedCost` per execution/session.
- **Stateful Tracking**: Accumulate usage across multiple turns in a workflow.
- **Fail-Safe**: If the limit is breached, the agent should halt immediately and return a safe error state (e.g., `CostLimitExceededException`), preventing further LLM calls.

**Describe alternatives you've considered**
- Manually tracking usage in every step of the workflow (error-prone and verbose).
- Setting strict `max_iterations` (imprecise; a single iteration with 128k context is different from 10 iterations of 1k).

**Additional context**
This feature is critical for "set and forget" autonomous scenarios in enterprise environments.