Improve postinstall message to detect existing Chrome installations (#815)

* Improve postinstall message to detect existing Chrome installations

Previously, the npm postinstall script always recommended running `agent-browser install` to download Chrome for Testing, even when users already had a working Chrome installation on their system.

This change adds Chrome detection logic to the postinstall script that mirrors the runtime behavior:

- Checks for system Chrome installations on macOS, Linux, and Windows
- Shows a success message when Chrome is found, indicating it will be used automatically
- Only shows the `agent-browser install` warning when no Chrome is detected
- Provides platform-specific guidance (Linux `--with-deps` flag, `--executable-path` alternative)

The detection logic matches the existing Rust `find_chrome()` implementation to ensure consistency between postinstall messaging and runtime behavior.

Fixes #814

* Mention --cdp, --provider, --engine as alternatives in postinstall message

---------

Co-authored-by: ctate <366502+ctate@users.noreply.github.com>

scripts/postinstall.js

@@ -128,15 +128,60 @@ async function main() {
   showInstallReminder();
 }
 
+function findSystemChrome() {
+  const os = platform();
+  if (os === 'darwin') {
+    const candidates = [
+      '/Applications/Google Chrome.app/Contents/MacOS/Google Chrome',
+      '/Applications/Google Chrome Canary.app/Contents/MacOS/Google Chrome Canary',
+      '/Applications/Chromium.app/Contents/MacOS/Chromium',
+    ];
+    return candidates.find(p => existsSync(p)) || null;
+  }
+  if (os === 'linux') {
+    const names = ['google-chrome', 'google-chrome-stable', 'chromium-browser', 'chromium'];
+    for (const name of names) {
+      try {
+        const result = execSync(`which ${name} 2>/dev/null`, { encoding: 'utf8' }).trim();
+        if (result) return result;
+      } catch {}
+    }
+    return null;
+  }
+  if (os === 'win32') {
+    const candidates = [
+      `${process.env.LOCALAPPDATA}\\Google\\Chrome\\Application\\chrome.exe`,
+      'C:\\Program Files\\Google\\Chrome\\Application\\chrome.exe',
+      'C:\\Program Files (x86)\\Google\\Chrome\\Application\\chrome.exe',
+    ];
+    return candidates.find(p => p && existsSync(p)) || null;
+  }
+  return null;
+}
+
 function showInstallReminder() {
+  const systemChrome = findSystemChrome();
+  if (systemChrome) {
+    console.log('');
+    console.log(`  ✓ System Chrome found: ${systemChrome}`);
+    console.log('    agent-browser will use it automatically.');
+    console.log('');
+    return;
+  }
+
   console.log('');
-  console.log('  To download Chrome, run:');
+  console.log('  ⚠ No Chrome installation detected.');
+  console.log('  If you plan to use a local browser, run:');
   console.log('');
   console.log('    agent-browser install');
+  if (platform() === 'linux') {
+    console.log('');
+    console.log('  On Linux, include system dependencies with:');
+    console.log('');
+    console.log('    agent-browser install --with-deps');
+  }
   console.log('');
-  console.log('  On Linux, include system dependencies with:');
-  console.log('');
-  console.log('    agent-browser install --with-deps');
+  console.log('  You can skip this if you use --cdp, --provider, --engine, or --executable-path.');
   console.log('');
 }