fix: enable shell mode for ACP agent spawn on Windows

On Windows, `npx` is a `.cmd` batch wrapper. Node's `spawn()` doesn't
resolve `.cmd` extensions, causing ENOENT even when npx is on PATH.
Adding `shell: true` on win32 lets cmd.exe handle the resolution.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: OpenSource03

electron/src/ipc/acp-sessions.ts

@@ -255,6 +255,7 @@ async function createAcpConnection(
   const proc = spawn(agentDef.binary, agentDef.args ?? [], {
     stdio: ["pipe", "pipe", "pipe"],
     env: { ...process.env, ...agentDef.env },
+    shell: process.platform === "win32",
   });
 
   // Process lifecycle handlers

package.json

@@ -1,6 +1,6 @@
 {
   "name": "harnss",
-  "version": "0.21.4",
+  "version": "0.21.5",
   "productName": "Harnss",
   "description": "Harness your AI coding agents — one desktop app for Claude Code, Codex, and any ACP agent",
   "author": {