fix(collab): treated guest prompts as user interjections

Converted user-attributed collab prompt frames to prioritized user messages only on the model-facing path, preserving guest details in persisted transcript frames.

Added regression coverage for the provider role, steering envelope, and retained guest attribution.

Fixes #7288

packages/coding-agent/test/session-messages.test.ts

@@ -7,6 +7,7 @@ import {
 	SKILL_PROMPT_MESSAGE_TYPE,
 	wrapSteeringForModel,
 } from "@oh-my-pi/pi-coding-agent/session/messages";
+import { COLLAB_PROMPT_MESSAGE_TYPE } from "@oh-my-pi/pi-wire";
 
 function expectAttribution(message: Message | undefined, expected: "user" | "agent" | undefined): void {
 	expect(message).toBeDefined();
@@ -382,6 +383,31 @@ describe("wrapSteeringForModel", () => {
 		expect(wrappedText).not.toContain("&");
 	});
 
+	it("presents user-attributed collab prompts as wrapped user turns", () => {
+		const message: AgentMessage = {
+			role: "custom",
+			customType: COLLAB_PROMPT_MESSAGE_TYPE,
+			content: "Reply with exactly PONG",
+			display: true,
+			details: { from: "guest" },
+			attribution: "user",
+			timestamp: 1,
+		};
+
+		const wrapped = wrapSteeringForModel([message]);
+		const providerMessages = convertToLlm(wrapped);
+
+		expect(wrapped[0]?.role).toBe("user");
+		expect(getUserText(wrapped[0])).toContain("<system-notice>");
+		expect(getUserText(wrapped[0])).toContain("Reply with exactly PONG");
+		expect(providerMessages).toHaveLength(1);
+		expect(providerMessages[0]?.role).toBe("user");
+		expect(message).toMatchObject({
+			role: "custom",
+			details: { from: "guest" },
+		});
+	});
+
 	it("wraps buried steering messages too so wire bytes stay stable across turns", () => {
 		const buried: AgentMessage = {
 			role: "user",