fix: isolate scratchpad session working directories

Give each scratchpad session its own working directory, migrate existing scratchpad sessions on workspace load, and route sidecar turns through the session-specific cwd. Add regression coverage for create, duplicate, delete, and migration flows, and document the new persistence model.

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

tests/main/workspaceRepositoryScratchpad.test.ts

@@ -0,0 +1,90 @@
+import { afterEach, beforeEach, describe, expect, mock, test } from 'bun:test';
+import { access, mkdir, readFile, rm, writeFile } from 'node:fs/promises';
+import { join } from 'node:path';
+
+import { createScratchpadProject } from '@shared/domain/project';
+import type { SessionRecord } from '@shared/domain/session';
+import { createWorkspaceSeed, type WorkspaceState } from '@shared/domain/workspace';
+
+const TIMESTAMP = '2026-03-28T00:00:00.000Z';
+const USER_DATA_PATH = 'C:\\workspace\\personal\\repositories\\aryx\\tests\\fixtures';
+
+mock.module('electron', () => {
+  const electronMock = {
+    app: {
+      getPath: () => USER_DATA_PATH,
+    },
+  };
+
+  return {
+    ...electronMock,
+    default: electronMock,
+  };
+});
+
+const { WorkspaceRepository } = await import('@main/persistence/workspaceRepository');
+
+function createStoredWorkspace(): WorkspaceState {
+  const workspace = createWorkspaceSeed();
+  const scratchpadProject = createScratchpadProject('C:\\legacy\\scratchpad', TIMESTAMP);
+  const patternId = workspace.patterns[0]?.id;
+  if (!patternId) {
+    throw new Error('Expected workspace seed to include at least one pattern.');
+  }
+
+  const session: SessionRecord = {
+    id: 'session-scratchpad',
+    projectId: scratchpadProject.id,
+    patternId,
+    title: 'Scratchpad',
+    createdAt: TIMESTAMP,
+    updatedAt: TIMESTAMP,
+    status: 'idle',
+    messages: [],
+    runs: [],
+  };
+
+  return {
+    ...workspace,
+    projects: [scratchpadProject],
+    sessions: [session],
+    selectedProjectId: scratchpadProject.id,
+    selectedPatternId: patternId,
+    selectedSessionId: session.id,
+  };
+}
+
+beforeEach(async () => {
+  await rm(join(USER_DATA_PATH, 'workspace.json'), { force: true });
+  await rm(join(USER_DATA_PATH, 'scratchpad'), { recursive: true, force: true });
+});
+
+afterEach(async () => {
+  await rm(join(USER_DATA_PATH, 'workspace.json'), { force: true });
+  await rm(join(USER_DATA_PATH, 'scratchpad'), { recursive: true, force: true });
+});
+
+describe('WorkspaceRepository scratchpad migration', () => {
+  test('assigns per-session scratchpad directories while loading existing workspace state', async () => {
+    const workspaceFilePath = join(USER_DATA_PATH, 'workspace.json');
+    await mkdir(USER_DATA_PATH, { recursive: true });
+    await writeFile(workspaceFilePath, JSON.stringify(createStoredWorkspace(), null, 2), 'utf8');
+
+    const repository = new WorkspaceRepository();
+    const loaded = await repository.load();
+    const loadedSession = loaded.sessions[0];
+    if (!loadedSession) {
+      throw new Error('Expected load() to return the migrated scratchpad session.');
+    }
+
+    const expectedScratchpadPath = join(USER_DATA_PATH, 'scratchpad');
+    const expectedSessionPath = join(expectedScratchpadPath, loadedSession.id);
+
+    expect(loaded.projects[0]?.path).toBe(expectedScratchpadPath);
+    expect(loadedSession.cwd).toBe(expectedSessionPath);
+    await access(expectedSessionPath);
+
+    const persisted = JSON.parse(await readFile(workspaceFilePath, 'utf8')) as WorkspaceState;
+    expect(persisted.sessions[0]?.cwd).toBe(expectedSessionPath);
+  });
+});