[M4] Implement Agent Card discovery and GetExtendedAgentCard

[rdobrik]

Description

Expose /.well-known/agent-card.json and protocol method for extended card retrieval, including declared capabilities and auth schemes.

Tasks

• Build AgentCardCatalog and card builder.
• Add HTTP route for /.well-known/agent-card.json.
• Implement GetExtendedAgentCard.
• Add extension points for JWS signing/verification and policy checks.

Acceptance Criteria

• Discovery endpoint returns valid card JSON.
• Extended card method returns additional fields as expected.
• Card capability/security sections are documented and test-covered.

Dependencies

• Issues 5, 6