[Architecture Review][P1] A2A must not form a parallel Runtime

[qqeasonchen]

Execution meta — see plan: #5296

• Phase: 4 — boundary decoupling
• Severity: P1
• Depends on: [Architecture Review][P0] Unified state control-plane stores #5301, [Architecture Review][P0] Decouple eventmesh-runtime responsibilities via internal packages #5297
• Blocks: (none)

Problem

A2A currently forms a parallel stack with its own Gateway, TaskRegistry, AgentCard, in-memory transport, and SSE task stream, largely independent of the Runtime.

Proposed direction

• A2A should be only a FrameAdaptor + a Task domain service.
• Publish, subscribe, retry, DLQ, authorization, and tracing must reuse the EventMesh Runtime.
• AgentCard should reuse the MetaStore / Registry.
• Until A2A is wired to a real Runtime transport and a recoverable TaskStore, it should stay Experimental or Beta — not be treated as a complete production path.

Acceptance criteria

• A2A transport is implemented over the Runtime delivery path, not a parallel in-memory transport.
• A2A tasks persist through TaskStore and recover across restarts.
• A2A publish/subscribe goes through the same reliability + DLQ + auth chain as HTTP/CloudEvents.
• AgentCard is stored/read via MetaStore or the shared registry.
• Documentation marks A2A as Experimental/Beta until the above lands.

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Part of the Architecture Review.