feat(browse): support Chrome extensions via BROWSE_EXTENSIONS_DIR

When the BROWSE_EXTENSIONS_DIR environment variable is set to a path
containing an unpacked Chrome extension, browse launches Chromium in
headed mode with the window off-screen (simulating headless) and loads
the extension.

This enables use cases like ad blockers (reducing token waste from
ad-heavy pages), accessibility tools, and custom request header
management — all while maintaining the same CLI interface.

Implementation:
- Read BROWSE_EXTENSIONS_DIR env var in launch()
- When set: switch to headed mode with --window-position=-9999,-9999
  (extensions require headed Chromium)
- Pass --load-extension and --disable-extensions-except to Chromium
- When unset: behavior is identical to before (headless, no extensions)

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

browse/src/browser-manager.ts

@@ -62,7 +62,28 @@ export class BrowserManager {
   private consecutiveFailures: number = 0;
 
   async launch() {
-    this.browser = await chromium.launch({ headless: true });
+    // ─── Extension Support ────────────────────────────────────
+    // BROWSE_EXTENSIONS_DIR points to an unpacked Chrome extension directory.
+    // Extensions only work in headed mode, so we use an off-screen window.
+    const extensionsDir = process.env.BROWSE_EXTENSIONS_DIR;
+    const launchArgs: string[] = [];
+    let useHeadless = true;
+
+    if (extensionsDir) {
+      launchArgs.push(
+        `--disable-extensions-except=${extensionsDir}`,
+        `--load-extension=${extensionsDir}`,
+        '--window-position=-9999,-9999',
+        '--window-size=1,1',
+      );
+      useHeadless = false; // extensions require headed mode; off-screen window simulates headless
+      console.log(`[browse] Extensions loaded from: ${extensionsDir}`);
+    }
+
+    this.browser = await chromium.launch({
+      headless: useHeadless,
+      ...(launchArgs.length > 0 ? { args: launchArgs } : {}),
+    });
 
     // Chromium crash → exit with clear message
     this.browser.on('disconnected', () => {