feat(core): add ERC-8004 Rust modules for agent identity, reputation, and validation

Implement three new core modules for ERC-8004 integration:

- agent_identity: AgentIdentityManager, registration file generation, ABI encoding
- reputation: FeedbackSignal, ReputationManager, feedback submission/querying
- validation: TrustModel enum, ValidationManager, TEE/zkML attestation support

Author: Kazopl