fix(coding-agent): parse multiline prompt template args

closes #4553

packages/coding-agent/test/prompt-templates.test.ts

@@ -13,7 +13,12 @@ import { tmpdir } from "os";
 import { join } from "path";
 import { afterAll, describe, expect, test } from "vitest";
 import { getAgentDir } from "../src/config.js";
-import { loadPromptTemplates, parseCommandArgs, substituteArgs } from "../src/core/prompt-templates.js";
+import {
+	expandPromptTemplate,
+	loadPromptTemplates,
+	parseCommandArgs,
+	substituteArgs,
+} from "../src/core/prompt-templates.js";
 
 // ============================================================================
 // substituteArgs
@@ -335,10 +340,25 @@ describe("parseCommandArgs", () => {
 		expect(parseCommandArgs("日本語 🎉 café")).toEqual(["日本語", "🎉", "café"]);
 	});
 
-	test("should handle newlines in arguments", () => {
+	test("should handle newlines in quoted arguments", () => {
 		expect(parseCommandArgs('"line1\nline2" second')).toEqual(["line1\nline2", "second"]);
 	});
 
+	test("should treat unquoted newlines as separators", () => {
+		expect(parseCommandArgs("label-2\n\nHere is some description #2.")).toEqual([
+			"label-2",
+			"Here",
+			"is",
+			"some",
+			"description",
+			"#2.",
+		]);
+	});
+
+	test("should collapse mixed unquoted whitespace", () => {
+		expect(parseCommandArgs("a\n\n\tb  c")).toEqual(["a", "b", "c"]);
+	});
+
 	test("should handle escaped quotes inside quoted strings", () => {
 		// Note: This implementation doesn't handle escaped quotes - backslash is literal
 		expect(parseCommandArgs('"quoted \\"text\\""')).toEqual(["quoted \\text\\"]);
@@ -357,6 +377,40 @@ describe("parseCommandArgs", () => {
 // Integration
 // ============================================================================
 
+describe("expandPromptTemplate", () => {
+	test("should split template arguments on unquoted newlines", () => {
+		const result = expandPromptTemplate("/arg-test label-2\n\nHere is some description #2.", [
+			{
+				name: "arg-test",
+				description: "test",
+				content: `- arg1: $1\n- rest: \${@:2}`,
+				sourceInfo: { path: "/tmp/arg-test.md", source: "local", scope: "temporary", origin: "top-level" },
+				filePath: "/tmp/arg-test.md",
+			},
+		]);
+
+		expect(result).toBe("- arg1: label-2\n- rest: Here is some description #2.");
+	});
+
+	test("should support template command separated from args by newline", () => {
+		const result = expandPromptTemplate("/arg-test\nlabel-2", [
+			{
+				name: "arg-test",
+				description: "test",
+				content: "arg1: $1",
+				sourceInfo: { path: "/tmp/arg-test.md", source: "local", scope: "temporary", origin: "top-level" },
+				filePath: "/tmp/arg-test.md",
+			},
+		]);
+
+		expect(result).toBe("arg1: label-2");
+	});
+});
+
+// ============================================================================
+// Integration
+// ============================================================================
+
 describe("parseCommandArgs + substituteArgs integration", () => {
 	test("should parse and substitute together correctly", () => {
 		const input = 'Button "onClick handler" "disabled support"';