chore(vendor): import pi 1.3.0

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: 71023aec1546d6d677a65669c5665c0927ce0dd0

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

@@ -205,6 +205,7 @@ describe("AgentSessionRuntime characterization", () => {
 			.getEntries()
 			.filter((entry) => entry.type === "message");
 		expect(outgoingEntries.map((entry) => entry.message.role)).toEqual([
+			"system",
 			"user",
 			"assistant",
 			"toolResult",
@@ -383,7 +384,7 @@ describe("AgentSessionRuntime characterization", () => {
 		expect(leafId).toBeTruthy();
 
 		await expect(runtime.fork(leafId!, { position: "at" })).rejects.toThrow(
-			"This session has not been saved yet. Wait for the first assistant response before cloning or forking it.",
+			"This session has not been saved yet. Send a message before cloning or forking it.",
 		);
 	});