feat(core-runner-protocol): scaffold protocol package

Hand-scaffolded from the packages/core-events shape: the core-package
turbo generator only covers its eight pre-curated snapshot names
(analytics, audit, consent, dsr, events, realtime, trpc, ui) and
rejects 'runner-protocol' at the name prompt. Flagged in the package
AGENTS.md as a future generator snapshot candidate.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/core-runner-protocol/src/index.ts

@@ -0,0 +1 @@
+export { PROTOCOL_VERSION, type ProtocolVersion } from "./protocol-version";

packages/core-runner-protocol/src/protocol-version.test.ts

@@ -0,0 +1,8 @@
+import { describe, it, expect } from "vitest";
+import { PROTOCOL_VERSION } from "@/protocol-version";
+
+describe("PROTOCOL_VERSION", () => {
+  it('is pinned to "0"', () => {
+    expect(PROTOCOL_VERSION).toBe("0");
+  });
+});

packages/core-runner-protocol/src/protocol-version.ts

@@ -0,0 +1,10 @@
+/**
+ * The pinned runner-protocol version (ADR-027).
+ *
+ * Every envelope is validated against this exact literal. Version
+ * negotiation beyond the pinned "0" is deliberately out of scope for the
+ * walking skeleton (later PRDs).
+ */
+export const PROTOCOL_VERSION = "0" as const;
+
+export type ProtocolVersion = typeof PROTOCOL_VERSION;