Add gas budget tracker with configurable limits

[abhicris]

Track cumulative gas spent per wallet. Configurable max per hour/day. Pause execution when budget exhausted. Include pytest tests. Difficulty: Beginner.

[D2758695161]

PR Ready!

Implemented at #10

Closes #5

[ledgerpilot]

Submitted PR #13 implementing the gas budget tracker with:

• GasTracker singleton class with configurable hourly and daily gas limits
• Thread-safe budget tracking with automatic hourly/daily resets
• can_send_transaction(gas) guard method
• GasBudgetExhaustedError exception for limit events
• Full pytest coverage (281 lines) including reset timing and configurable limits

Happy to adjust limits, add callback hooks, or integrate with the existing nonce manager if useful.