feat(companion): support default canvas session in platform clients

src/companion/platformClients.test.ts

@@ -183,4 +183,44 @@ describe('platform companion clients', () => {
       }),
     );
   });
+
+  it('uses defaultSessionId for canvas operations when sessionId is omitted', async () => {
+    const mock = createRuntimeMock();
+    const client = new IOSCompanionClient({
+      runtime: mock.runtime,
+      nodeId: 'ios-node',
+      defaultSessionId: 'ws:default-canvas',
+    });
+
+    await client.putCanvasArtifact({
+      type: 'markdown',
+      content: { body: 'default session' },
+    });
+    await client.listCanvasArtifacts();
+    await client.getCanvasArtifact({ artifactId: 'a1' });
+    await client.deleteCanvasArtifact({ artifactId: 'a1' });
+    await client.clearCanvasArtifacts();
+
+    expect(mock.putCanvasArtifact).toHaveBeenCalledWith({
+      sessionId: 'ws:default-canvas',
+      type: 'markdown',
+      content: { body: 'default session' },
+      artifactId: undefined,
+      title: undefined,
+      metadata: undefined,
+    });
+    expect(mock.listCanvasArtifacts).toHaveBeenCalledWith('ws:default-canvas');
+    expect(mock.getCanvasArtifact).toHaveBeenCalledWith({ sessionId: 'ws:default-canvas', artifactId: 'a1' });
+    expect(mock.deleteCanvasArtifact).toHaveBeenCalledWith({ sessionId: 'ws:default-canvas', artifactId: 'a1' });
+    expect(mock.clearCanvasArtifacts).toHaveBeenCalledWith('ws:default-canvas');
+  });
+
+  it('throws when canvas sessionId is missing and no defaultSessionId is configured', async () => {
+    const mock = createRuntimeMock();
+    const client = new MacOSCompanionClient({ runtime: mock.runtime, nodeId: 'mac-node' });
+
+    expect(() => client.listCanvasArtifacts()).toThrow(
+      'sessionId is required (provide one or configure defaultSessionId)',
+    );
+  });
 });