Define AI Registry Federation Protocol (RFP) Specification

## Summary

Define a formal protocol specification for peer-to-peer registry federation, enabling any AI registry implementation to participate in federated discovery regardless of their underlying technology stack.

## Background

We currently have a working federation API implementation, but it exists only as code. To enable interoperability with other registry implementations, we need to extract and formalize this into a **protocol specification** that can be implemented by any registry.

## Why This Matters

| Reason | Benefit |
|--------|---------|
| **Interoperability** | Other AI registries (community or commercial) could federate with us if they implement the same protocol |
| **Standardization** | Aligns with the broader AI protocol ecosystem (such as MCP, A2A) which are defining formal specs |
| **Documentation** | A spec serves as authoritative documentation for our own federation features |
| **Community** | Opens the door for community contributions and alternative implementations |

## Protocol Overview

The federation protocol would define:

| Component | Description |
|-----------|-------------|
| **Registry Card** | Discovery document (like A2A's Agent Card) at `/.well-known/rfp` |
| **Export Operations** | How registries expose servers/agents/skills/prompts for federation |
| **Sync Semantics** | Generation-based incremental sync, orphan handling, chain prevention |
| **Authentication** | OAuth2/OIDC and static bearer token support |
| **Push Notifications** | Optional webhooks for real-time updates (v1.1) |

## Alignment with MCP and A2A

The specification would follow patterns established by:

- **MCP** (Model Context Protocol) - RFC 2119 normative language, JSON Schema
- **A2A** (Agent-to-Agent Protocol) - Layered architecture (Data Model, Operations, Bindings)

```
┌───────────────┐ ┌─────────────┐ ┌─────────────────┐
│ MCP │ │ A2A │ │ AI Registry │
│ Agent ↔ Tools │ │Agent ↔ Agent│ │ Federation │
│ │ │ │ │Registry↔Registry│
└───────────────┘ └─────────────┘ └─────────────────┘
```

## Deliverables

- [ ] Protocol specification document (RFC-style markdown)
- [ ] JSON Schemas for all data models
- [ ] Sequence diagrams for key flows
- [ ] Compliance checklist for implementers
- [ ] Update existing API to serve Registry Card at well-known endpoint

## Sync Model

- **v1.0**: Pull-based (registries poll peers on schedule)
- **v1.1**: Optional push via webhooks for real-time notifications

## References

- [MCP Specification](https://modelcontextprotocol.io/specification)
- [A2A Protocol Specification](https://a2a-protocol.org/latest/specification/)
- Current implementation: `registry/api/federation_export_routes.py`, `registry/api/peer_management_routes.py`