Add New Diagram command with starter templates

- Archify: New Diagram… (Explorer folder menu, File > New File…, Command
  Palette) creates a .archify file from a template and opens the diagram
- Six YAML templates: architecture, workflow, sequence, dataflow,
  lifecycle and an everyday plan; connections carry ids, workflows use
  schema 2
- Title and meta.output follow the file name; template comments are kept
- Tests: every template passes archify validate; integration check for
  the command

Closes #2

test/integration/suite.ts

@@ -97,6 +97,35 @@ export async function run(): Promise<void> {
     assert.ok(diagnostics[0].range.start.line >= 2, `reported on line ${diagnostics[0].range.start.line + 1}`);
   });
 
+  await check('New Diagram creates a .archify file from a template and opens the diagram', async () => {
+    const folder = workspaceFile('.');
+    // Stub the template pick and the name prompt; check the name validation on the way.
+    const window = vscode.window as unknown as Record<string, unknown>;
+    const originalPick = window.showQuickPick;
+    const originalInput = window.showInputBox;
+    let existingNameMessage: unknown;
+    window.showQuickPick = async (items: readonly { label: string }[]) => (await items).find((item) => item.label === 'Everyday Plan');
+    window.showInputBox = async (options: vscode.InputBoxOptions) => {
+      existingNameMessage = await options.validateInput?.('web-platform');
+      return 'summer-plan';
+    };
+    try {
+      await vscode.commands.executeCommand('archify.newDiagram', folder);
+    } finally {
+      window.showQuickPick = originalPick;
+      window.showInputBox = originalInput;
+    }
+    assert.match(String(existingNameMessage), /already exists/);
+    const uri = vscode.Uri.joinPath(folder, 'summer-plan.archify');
+    const text = new TextDecoder().decode(await vscode.workspace.fs.readFile(uri));
+    assert.match(text, /^diagram_type: workflow$/m);
+    assert.match(text, /^ {2}title: Summer plan$/m);
+    assert.match(text, /^ {2}output: summer-plan\.html$/m);
+    await waitFor('diagram editor for the new file', () =>
+      webviewTabs('archify.diagram').some((tab) => tab.input instanceof vscode.TabInputCustom && tab.input.uri.toString() === uri.toString()),
+    );
+  });
+
   await check('opens a live preview beside the source', async () => {
     await vscode.commands.executeCommand('archify.showPreviewToSide', workspaceFile('production-deployment.architecture.json'));
     const tabs = await waitFor('preview tab', () => {