feat: add scratchpad chat sessions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/shared/project.test.ts

@@ -0,0 +1,46 @@
+import { describe, expect, test } from 'bun:test';
+
+import {
+  SCRATCHPAD_PROJECT_ID,
+  SCRATCHPAD_PROJECT_NAME,
+  createScratchpadProject,
+  isScratchpadProject,
+  mergeScratchpadProject,
+} from '@shared/domain/project';
+
+describe('scratchpad project helpers', () => {
+  test('creates a stable built-in scratchpad project', () => {
+    const project = createScratchpadProject('C:\\Users\\me\\AppData\\Roaming\\kopaya\\scratchpad');
+
+    expect(project.id).toBe(SCRATCHPAD_PROJECT_ID);
+    expect(project.name).toBe(SCRATCHPAD_PROJECT_NAME);
+    expect(project.path).toContain('scratchpad');
+  });
+
+  test('recognizes scratchpad project ids and records', () => {
+    expect(isScratchpadProject(SCRATCHPAD_PROJECT_ID)).toBe(true);
+    expect(
+      isScratchpadProject({
+        id: SCRATCHPAD_PROJECT_ID,
+      }),
+    ).toBe(true);
+    expect(isScratchpadProject('project-normal')).toBe(false);
+  });
+
+  test('merges scratchpad project ahead of normal user projects and preserves user projects', () => {
+    const merged = mergeScratchpadProject(
+      [
+        {
+          id: 'project-a',
+          name: 'Repo A',
+          path: 'C:\\repo-a',
+          addedAt: '2026-03-23T00:00:00.000Z',
+        },
+      ],
+      'C:\\Users\\me\\AppData\\Roaming\\kopaya\\scratchpad',
+    );
+
+    expect(merged[0].id).toBe(SCRATCHPAD_PROJECT_ID);
+    expect(merged[1].id).toBe('project-a');
+  });
+});