feat(web): brave_free plugin (first migration from tools/web_providers/)

Adds plugins/web/brave_free/ as the first plugin built against the new
WebSearchProvider ABC. Mirrors the plugins/image_gen/openai/ layout exactly:

  plugins/web/brave_free/
    plugin.yaml      kind: backend, provides_web_providers: [brave-free]
    __init__.py      register(ctx) -> ctx.register_web_search_provider(...)
    provider.py      BraveFreeWebSearchProvider(WebSearchProvider)

Behavior preserved: same name ("brave-free" with hyphen), same env var
(BRAVE_SEARCH_API_KEY), same HTTP request shape, same response normalization.

The legacy tools/web_providers/brave_free.py is left in place — the
dispatcher in tools/web_tools.py still references it. Task 7 cuts over the
dispatcher to the new registry; Task 10 deletes the legacy file.

E2E verified:
  HERMES_PLUGINS_DEBUG=1 python -c "
  from hermes_cli.plugins import _ensure_plugins_discovered
  _ensure_plugins_discovered()
  from agent.web_search_registry import list_providers
  print([p.name for p in list_providers()])
  "
  # -> ['brave-free']

plugins/web/brave_free/__init__.py

@@ -0,0 +1,14 @@
+"""Brave Search (free tier) plugin — bundled, auto-loaded.
+
+Mirrors the ``plugins/image_gen/openai/`` layout: ``provider.py`` holds the
+provider class, ``__init__.py::register(ctx)`` registers an instance.
+"""
+
+from __future__ import annotations
+
+from plugins.web.brave_free.provider import BraveFreeWebSearchProvider
+
+
+def register(ctx) -> None:
+    """Register the Brave-free provider with the plugin context."""
+    ctx.register_web_search_provider(BraveFreeWebSearchProvider())