fix(coding-agent): preserve pi.exec spawn errors

Fixes #11085

packages/coding-agent/src/core/exec.ts

@@ -96,12 +96,12 @@ export async function execCommand(
 				}
 				resolve({ stdout, stderr, code: code ?? 0, killed });
 			})
-			.catch((_err) => {
+			.catch((error: Error) => {
 				if (timeoutId) clearTimeout(timeoutId);
 				if (options?.signal) {
 					options.signal.removeEventListener("abort", killProcess);
 				}
-				resolve({ stdout, stderr, code: 1, killed });
+				resolve({ stdout, stderr: stderr || error.message, code: 1, killed });
 			});
 	});
 }

packages/coding-agent/test/exec.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from "vitest";
+import { execCommand } from "../src/core/exec.ts";
+
+describe("execCommand", () => {
+	// Regression test for https://github.com/earendil-works/pi/issues/11085
+	it("reports spawn errors in stderr", async () => {
+		const result = await execCommand("pi-command-that-does-not-exist-11085", [], process.cwd());
+
+		expect(result).toMatchObject({
+			code: 1,
+			killed: false,
+			stdout: "",
+		});
+		expect(result.stderr).toMatch(/ENOENT/);
+	});
+
+	it("keeps stderr empty when a command exits silently", async () => {
+		const result = await execCommand(process.execPath, ["-e", "process.exit(1)"], process.cwd());
+
+		expect(result).toEqual({ stdout: "", stderr: "", code: 1, killed: false });
+	});
+});