feat: A2A protocol support — agent-to-agent communication

[rfunix]

Context

A2A (Agent2Agent) is Google's open protocol for agent interoperability, now under Linux Foundation. Kōdo has MCP but not A2A.

Proposal

1. Generate agent-card.json from Kōdo module meta blocks
2. kodoc a2a command to serve agent card
3. Map @confidence and contracts to A2A capability descriptions

Research

See docs/research/2026-03-29-idris-qtt-and-a2a-protocol.md