Proposal: Implement A2A Agent Registrar for Curated Agent Discovery(Registry-Based)

I propose implementing a Agent Registrar to facilitate curated discovery of agents across organizations. This would provide a centralized mechanism for agent registration, discovery, and management while respecting organizational boundaries and visibility controls.

PR: https://github.com/google/A2A/pull/296

### Benefits

- Enterprise-grade agent discovery with organizational boundaries
- Standardized registry interface for consistent agent discovery
- Controlled visibility for sensitive or internal-only agents
- Authentication model preserving security of agent registrations

The issue proposes a new Agent Registrar system for agent discovery. Review the linked PR #296 for the proposed implementation details and design. Understand the existing A2A protocol architecture and agent communication patterns. Determine the components needed for registration, discovery interfaces, and organizational visibility controls.

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