[Architecture Review][P2] Sync documentation with implementation status

[qqeasonchen]

Execution meta — see plan: #5296

• Phase: 4 — documentation (throughout)
• Severity: P2
• Depends on: (none)
• Blocks: (none)

Problem

README, redesign docs, A2A docs, and production-readiness docs describe different architectural stages. Some modules still say "to be re-implemented" or "legacy compatibility", but the user-facing entry points are not clearly marked.

Proposed direction

Maintain a per-module / per-protocol status table:

Capability	Status	Recommendation	Migration target
HTTP + CloudEvents	GA target	Recommended	Primary path
Kafka / RocketMQ Storage	GA target	Recommended	TCK-covered
SSE / WebSocket	Beta	Usable	Unified ACK/redelivery
Connector Runtime	Beta	Usable	New SPI migration
A2A / Agent	Experimental	Evaluate	Unified Runtime
TCP / gRPC / OpenMessaging	Legacy-compatible	Existing users only	Separate legacy artifact

Acceptance criteria

• A single, linked status table is maintained in the main README.
• Each module's docs state its status, recommendation, and migration target consistently.
• "To be re-implemented" / "legacy" notes point to the status table.

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Part of the Architecture Review.