[Good First Issue] Add friendly validation for custom pricing JSON

Description When a user supplies a custom pricing card via --pricing '' or AGENT_COST_PRICING, if a key or number format is invalid, we should provide clear, human-readable error messages pointing to the exact offending key. Requirements Validate that rate card entries have input, output, cache_read, cache_write (or standard subsets). Raise a friendly CLI error if values are non-numeric or negative. Add unit tests in tests/test_pricing.py. Good first issue for anyone looking to contribute!