fix: Convert sync file I/O to async in agent spawn path to unblock event loop

writeInputFiles, spawnDetached, and diagnostic writes now use
fs/promises (mkdir, writeFile) instead of mkdirSync/writeFileSync.
File writes in writeInputFiles are batched with Promise.all.
openSync/closeSync for child process stdio FDs remain sync as
spawn() requires the FDs immediately.

apps/server/agent/file-io.ts

@@ -8,8 +8,8 @@
  * Output: .cw/output/ — written by agent during execution
  */
 
-import { mkdirSync, writeFileSync, readdirSync, existsSync } from 'node:fs';
-import { readFileSync } from 'node:fs';
+import { readdirSync, existsSync, readFileSync } from 'node:fs';
+import { mkdir, writeFile } from 'node:fs/promises';
 import { join } from 'node:path';
 import matter from 'gray-matter';
 import { nanoid } from 'nanoid';
@@ -109,12 +109,12 @@ function formatFrontmatter(data: Record<string, unknown>, body: string = ''): st
   return lines.join('\n') + '\n';
 }
 
-export function writeInputFiles(options: WriteInputFilesOptions): void {
+export async function writeInputFiles(options: WriteInputFilesOptions): Promise<void> {
   const inputDir = join(options.agentWorkdir, '.cw', 'input');
-  mkdirSync(inputDir, { recursive: true });
+  await mkdir(inputDir, { recursive: true });
 
   // Write expected working directory marker for verification
-  writeFileSync(
+  await writeFile(
     join(inputDir, '../expected-pwd.txt'),
     options.agentWorkdir,
     'utf-8'
@@ -122,6 +122,9 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
 
   const manifestFiles: string[] = [];
 
+  // Collect all file writes, then flush in parallel
+  const writes: Array<{ path: string; content: string }> = [];
+
   if (options.initiative) {
     const ini = options.initiative;
     const content = formatFrontmatter(
@@ -134,13 +137,12 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
       },
       '',
     );
-    writeFileSync(join(inputDir, 'initiative.md'), content, 'utf-8');
+    writes.push({ path: join(inputDir, 'initiative.md'), content });
     manifestFiles.push('initiative.md');
   }
 
   if (options.pages && options.pages.length > 0) {
-    const pagesDir = join(inputDir, 'pages');
-    mkdirSync(pagesDir, { recursive: true });
+    await mkdir(join(inputDir, 'pages'), { recursive: true });
 
     for (const page of options.pages) {
       let bodyMarkdown = '';
@@ -162,7 +164,7 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
         bodyMarkdown,
       );
       const filename = `pages/${page.id}.md`;
-      writeFileSync(join(pagesDir, `${page.id}.md`), content, 'utf-8');
+      writes.push({ path: join(inputDir, 'pages', `${page.id}.md`), content });
       manifestFiles.push(filename);
     }
   }
@@ -185,7 +187,7 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
       },
       bodyMarkdown,
     );
-    writeFileSync(join(inputDir, 'phase.md'), content, 'utf-8');
+    writes.push({ path: join(inputDir, 'phase.md'), content });
     manifestFiles.push('phase.md');
   }
 
@@ -202,7 +204,7 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
       },
       t.description ?? '',
     );
-    writeFileSync(join(inputDir, 'task.md'), content, 'utf-8');
+    writes.push({ path: join(inputDir, 'task.md'), content });
     manifestFiles.push('task.md');
   }
 
@@ -210,8 +212,7 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
   const contextFiles: string[] = [];
 
   if (options.phases && options.phases.length > 0) {
-    const phasesDir = join(inputDir, 'context', 'phases');
-    mkdirSync(phasesDir, { recursive: true });
+    await mkdir(join(inputDir, 'context', 'phases'), { recursive: true });
 
     for (const ph of options.phases) {
       let bodyMarkdown = '';
@@ -232,14 +233,13 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
         bodyMarkdown,
       );
       const filename = `context/phases/${ph.id}.md`;
-      writeFileSync(join(phasesDir, `${ph.id}.md`), content, 'utf-8');
+      writes.push({ path: join(inputDir, 'context', 'phases', `${ph.id}.md`), content });
       contextFiles.push(filename);
     }
   }
 
   if (options.tasks && options.tasks.length > 0) {
-    const tasksDir = join(inputDir, 'context', 'tasks');
-    mkdirSync(tasksDir, { recursive: true });
+    await mkdir(join(inputDir, 'context', 'tasks'), { recursive: true });
 
     for (const t of options.tasks) {
       const content = formatFrontmatter(
@@ -257,7 +257,7 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
         t.description ?? '',
       );
       const filename = `context/tasks/${t.id}.md`;
-      writeFileSync(join(tasksDir, `${t.id}.md`), content, 'utf-8');
+      writes.push({ path: join(inputDir, 'context', 'tasks', `${t.id}.md`), content });
       contextFiles.push(filename);
     }
   }
@@ -276,17 +276,18 @@ export function writeInputFiles(options: WriteInputFilesOptions): void {
         status: t.status,
       });
     }
-    const contextDir = join(inputDir, 'context');
-    mkdirSync(contextDir, { recursive: true });
-    writeFileSync(
-      join(contextDir, 'index.json'),
-      JSON.stringify({ tasksByPhase }, null, 2) + '\n',
-      'utf-8',
-    );
+    await mkdir(join(inputDir, 'context'), { recursive: true });
+    writes.push({
+      path: join(inputDir, 'context', 'index.json'),
+      content: JSON.stringify({ tasksByPhase }, null, 2) + '\n',
+    });
   }
 
-  // Write manifest listing exactly which files were created
-  writeFileSync(
+  // Flush all file writes in parallel — yields the event loop between I/O ops
+  await Promise.all(writes.map(w => writeFile(w.path, w.content, 'utf-8')));
+
+  // Write manifest last (after all files exist)
+  await writeFile(
     join(inputDir, 'manifest.json'),
     JSON.stringify({
       files: manifestFiles,