feat(browser): browserbase plugin (spike — first migration)

Migrates tools/browser_providers/browserbase.py → plugins/browser/browserbase/.
Direct credentials only (BROWSERBASE_API_KEY + BROWSERBASE_PROJECT_ID); same
session-creation, 402-handling, and feature-flag logic as the legacy
implementation. Renames is_configured() → is_available() to match the new
BrowserProvider ABC.

The legacy module tools/browser_providers/browserbase.py is NOT yet deleted
and tools/browser_tool.py still references the in-tree class. The dispatcher
cutover happens in a later commit so the plugin migration and the dispatcher
switch land as separate reviewable units.

Verified via plugin-discovery E2E:
  - browserbase registers as 'browserbase'
  - is_available() correctly tracks BROWSERBASE_API_KEY + BROWSERBASE_PROJECT_ID
  - _resolve('browserbase') returns the provider even when unavailable
    (so dispatcher surfaces a typed credentials error)
  - _resolve(None) returns the provider when it's the single eligible one

plugins/browser/browserbase/__init__.py

@@ -0,0 +1,15 @@
+"""Browserbase cloud browser plugin — bundled, auto-loaded.
+
+Mirrors the ``plugins/web/<vendor>/`` and ``plugins/image_gen/openai/``
+layout: ``provider.py`` holds the provider class; ``__init__.py::register``
+instantiates and registers it via the plugin context.
+"""
+
+from __future__ import annotations
+
+from plugins.browser.browserbase.provider import BrowserbaseBrowserProvider
+
+
+def register(ctx) -> None:
+    """Register the Browserbase provider with the plugin context."""
+    ctx.register_browser_provider(BrowserbaseBrowserProvider())