Add XCUITest authoring docs and reusable prompt template

docs/templates/XCUITestSuiteTemplate.swift

@@ -0,0 +1,34 @@
+//
+//  <Feature>Tests.swift
+//  Tests iOS
+//
+//  Replace placeholders and move into Tests iOS/ when creating a real suite.
+//
+
+import XCTest
+
+final class <Feature>Tests: BaseUITestCase {
+    // Choose fixture: "empty", "single_mood", "week_of_moods"
+    override var seedFixture: String? { "empty" }
+
+    // Override launch behavior only when needed for this feature.
+    // override var skipOnboarding: Bool { false }
+    // override var bypassSubscription: Bool { false }
+    // override var expireTrial: Bool { true }
+
+    func test<Behavior>() {
+        let tabBar = TabBarScreen(app: app)
+
+        // Navigate using screen objects.
+        let day = tabBar.tapDay()
+
+        // Interact using identifier-backed elements and helpers.
+        day.assertMoodHeaderVisible()
+
+        // Add an attachment for triage/debugging.
+        captureScreenshot(name: "<feature_behavior_state>")
+
+        // Assert outcome.
+        XCTAssertTrue(day.moodHeader.exists, "Expected day mood header to exist")
+    }
+}