[Feat] Agent TOML Parser & Validator

[abhipatel12]

Implement the loading and validation logic for the Agent Definition Spec.

Completion Criteria

• [ ] Create/settle on Agent Definition using TOML
• [ ] Define Zod schemas for AgentConfig (Metadata, Prompts, Inputs, Outputs, Model).
• [ ] Validation: specific error messages for invalid TOML (e.g., "Missing 'description' in agent 'foo'").