Add maxTokenBudget with automatic abort

Issue:

• There is currently no mechanism to enforce a budget or abort when spending exceeds a threshold.
• A single agent stuck in a tool loop can consume thousands of tokens per turn.
• Users running against paid APIs (e.g., Anthropic, OpenAI) have no safeguards against runaway costs.
• Implementing token budget enforcement would be high-value and low-effort.

Proposed implementation:

• Allow passing a max_token_budget configuration value.