Announcement + deprecation notice for repo split

[mojoatomic]

Goal
Announce the protocol/SDK repo split and steer users to the new protocol site.

Scope

• Add a short notice to rdcp README and wiki explaining the split.
• Optional GitHub Discussion post linking to https://rdcp.dev (protocol site) and https://github.com/mojoatomic/rdcp-protocol.
• Mention stable versioned URLs and where to find API/Specs/Compliance Kit.

Acceptance criteria

• README includes a clear deprecation/redirect notice.
• Wiki home page includes the same pointer.
• (If used) Discussion post created and linked.

Tracking

• Part of Separate protocol specification from SDK implementation into distinct repositories #111
• Protocol side: Compliance kit for cross-language self-certification rdcp-protocol#3, ci: build otel-plugin before tests; explicit plugin install #5, ci: build root and link @rdcp/server before building otel-plugin #6