[Feature]: Add prekey REST endpoints to AgentMesh Registry service

[imran-siddique]

The Registry service has agent registration and discovery endpoints but no prekey management:

• \POST /prekeys/{agentId}\ — publish prekey bundles
• \GET /prekeys/{agentId}\ — fetch prekey bundle (with atomic OPK consumption)

Without these, clients cannot establish encrypted sessions via the registry.

Ref: Azure/kars#27 review (blocker #7)

[imran-siddique]

Shipped in v3.2.0 — the AgentMesh Registry service has 8 REST endpoints including prekey bundle management. See \packages/agent-mesh/src/agentmesh/registry/app.py.