fix(coding-agent): split /clone from /fork UX

closes #2962

packages/coding-agent/test/suite/agent-session-runtime.test.ts

@@ -234,6 +234,158 @@ describe("AgentSessionRuntime characterization", () => {
 		expect(events).toEqual([{ type: "session_before_fork", entryId: "missing-entry", position: "at" }]);
 	});
 
+	it("duplicates the current active branch when forking at the current position", async () => {
+		const { runtime } = await createRuntimeForTest(() => {});
+		await runtime.session.prompt("hello");
+		await runtime.session.prompt("again");
+
+		const beforeMessages = runtime.session.messages.map((message) => ({
+			role: message.role,
+			text:
+				message.role === "user"
+					? typeof message.content === "string"
+						? message.content
+						: message.content
+								.filter((part): part is { type: "text"; text: string } => part.type === "text")
+								.map((part) => part.text)
+								.join("")
+					: undefined,
+		}));
+		const previousSessionFile = runtime.session.sessionFile;
+		const leafId = runtime.session.sessionManager.getLeafId();
+		expect(leafId).toBeTruthy();
+
+		const result = await runtime.fork(leafId!, { position: "at" });
+		expect(result).toEqual({ cancelled: false, selectedText: undefined });
+		expect(runtime.session.sessionFile).not.toBe(previousSessionFile);
+		expect(
+			runtime.session.messages.map((message) => ({
+				role: message.role,
+				text:
+					message.role === "user"
+						? typeof message.content === "string"
+							? message.content
+							: message.content
+									.filter((part): part is { type: "text"; text: string } => part.type === "text")
+									.map((part) => part.text)
+									.join("")
+						: undefined,
+			})),
+		).toEqual(beforeMessages);
+	});
+
+	it("duplicates the current active branch in-memory when forking at the current position", async () => {
+		const tempDir = join(tmpdir(), `pi-runtime-suite-in-memory-${Date.now()}-${Math.random().toString(36).slice(2)}`);
+		mkdirSync(tempDir, { recursive: true });
+
+		const faux = registerFauxProvider({
+			models: [
+				{ id: "faux-1", reasoning: true },
+				{ id: "faux-2", reasoning: false },
+			],
+		});
+		faux.setResponses([fauxAssistantMessage("one"), fauxAssistantMessage("two"), fauxAssistantMessage("three")]);
+
+		const authStorage = AuthStorage.inMemory();
+		authStorage.setRuntimeApiKey(faux.getModel().provider, "faux-key");
+
+		const runtimeOptions = {
+			agentDir: tempDir,
+			authStorage,
+			model: faux.getModel(),
+			resourceLoaderOptions: {
+				extensionFactories: [
+					(pi: ExtensionAPI) => {
+						pi.registerProvider(faux.getModel().provider, {
+							baseUrl: faux.getModel().baseUrl,
+							apiKey: "faux-key",
+							api: faux.api,
+							models: faux.models.map((registeredModel) => ({
+								id: registeredModel.id,
+								name: registeredModel.name,
+								api: registeredModel.api,
+								reasoning: registeredModel.reasoning,
+								input: registeredModel.input,
+								cost: registeredModel.cost,
+								contextWindow: registeredModel.contextWindow,
+								maxTokens: registeredModel.maxTokens,
+							})),
+						});
+					},
+				],
+				noSkills: true,
+				noPromptTemplates: true,
+				noThemes: true,
+			},
+		};
+		const createRuntime: CreateAgentSessionRuntimeFactory = async ({ cwd, sessionManager, sessionStartEvent }) => {
+			const services = await createAgentSessionServices({
+				...runtimeOptions,
+				cwd,
+			});
+			return {
+				...(await createAgentSessionFromServices({
+					services,
+					sessionManager,
+					sessionStartEvent,
+					model: runtimeOptions.model,
+				})),
+				services,
+				diagnostics: services.diagnostics,
+			};
+		};
+		const runtime = await createAgentSessionRuntime(createRuntime, {
+			cwd: tempDir,
+			agentDir: tempDir,
+			sessionManager: SessionManager.inMemory(tempDir),
+		});
+		await runtime.session.bindExtensions({});
+		cleanups.push(async () => {
+			await runtime.dispose();
+			faux.unregister();
+			if (existsSync(tempDir)) {
+				rmSync(tempDir, { recursive: true, force: true });
+			}
+		});
+
+		await runtime.session.prompt("hello");
+		await runtime.session.prompt("again");
+
+		const beforeMessages = runtime.session.messages.map((message) => ({
+			role: message.role,
+			text:
+				message.role === "user"
+					? typeof message.content === "string"
+						? message.content
+						: message.content
+								.filter((part): part is { type: "text"; text: string } => part.type === "text")
+								.map((part) => part.text)
+								.join("")
+					: undefined,
+		}));
+		const leafId = runtime.session.sessionManager.getLeafId();
+		expect(leafId).toBeTruthy();
+		expect(runtime.session.sessionFile).toBeUndefined();
+
+		const result = await runtime.fork(leafId!, { position: "at" });
+		expect(result).toEqual({ cancelled: false, selectedText: undefined });
+		expect(runtime.session.sessionFile).toBeUndefined();
+		expect(
+			runtime.session.messages.map((message) => ({
+				role: message.role,
+				text:
+					message.role === "user"
+						? typeof message.content === "string"
+							? message.content
+							: message.content
+									.filter((part): part is { type: "text"; text: string } => part.type === "text")
+									.map((part) => part.text)
+									.join("")
+						: undefined,
+			})),
+		).toEqual(beforeMessages);
+	});
+
 	it("throws when forking with an invalid entry id", async () => {
 		const { runtime } = await createRuntimeForTest(() => {});
 		await expect(runtime.fork("missing-entry")).rejects.toThrow("Invalid entry ID for forking");