refactor(web): remove legacy in-tree provider modules

Deletes tools/web_providers/{brave_free,ddgs,searxng}.py — the three
providers that moved to plugins/web/ in prior commits. tools/web_tools.py
no longer imports them (registry dispatch as of d8735963f), so removing
them is purely a cleanup pass.

Also migrates the existing tests to the new import paths:
  tests/tools/test_web_providers_brave_free.py
  tests/tools/test_web_providers_ddgs.py
  tests/tools/test_web_providers_searxng.py

Mechanical rewrites:
  - `from tools.web_providers.X import YSearchProvider`
      -> `from plugins.web.X.provider import YWebSearchProvider`
  - `.is_configured()` -> `.is_available()`        (legacy method  -> new method)
  - `.provider_name()` -> `.name`                  (legacy method  -> new property)
  - `from tools.web_providers.base import WebSearchProvider`
      -> `from agent.web_search_provider import WebSearchProvider`
      (the subclass-check asserts membership in the new plugin-facing ABC)
  - `sys.modules.delitem("tools.web_providers.ddgs")` updated to point at
    `plugins.web.ddgs.provider` (cache-busting for lazy ddgs imports)

The TestXBackendWiring / TestXSearchOnlyErrors classes (covering
_is_backend_available, _get_backend, check_web_api_key, and the
"search-only" error paths in web_extract/web_crawl) are untouched —
those still test web_tools.py's backend-selection logic, which continues
to recognize the names "brave-free" / "ddgs" / "searxng" even after the
modules behind them moved to plugins.

tools/web_providers/base.py is intentionally NOT deleted by this commit
— it's the parent ABC of the legacy modules and shares its name with
agent/web_search_provider.py::WebSearchProvider. Removing it surfaces the
naming collision (see PR description Finding 0); the real migration PR
deletes it in the same commit that drops the _WEB_PLUGIN_SKIPLIST
guards in hermes_cli/tools_config.py.

Test results:
  bash scripts/run_tests.sh tests/tools/test_web_providers_*.py
  -> 65 passed in 3.41s (all rewritten unit tests + unchanged integration tests)
  bash scripts/run_tests.sh tests/tools/test_web_*.py
  -> 141 passed in 4.70s (full web test set, post-deletion)

plugins/web/brave_free/provider.py

@@ -1,9 +1,9 @@
 """Brave Search (free tier) — plugin form.
 
 Subclasses :class:`agent.web_search_provider.WebSearchProvider` (the
-plugin-facing ABC) and reuses the existing Brave search logic from the
-legacy ``tools.web_providers.brave_free`` module. Once the spike validates
-the pattern, the legacy module is deleted and this becomes the canonical
+plugin-facing ABC). The legacy in-tree module
+``tools.web_providers.brave_free`` was removed in the same commit that
+moved this code under ``plugins/``; this file is now the canonical
 implementation.
 
 Config keys this provider responds to::

plugins/web/ddgs/provider.py

@@ -1,8 +1,9 @@
 """DuckDuckGo search — plugin form (via the ``ddgs`` package).
 
 Subclasses the plugin-facing :class:`agent.web_search_provider.WebSearchProvider`.
-Same behavior as the legacy ``tools.web_providers.ddgs`` module — only the
-ABC name and import path change.
+The legacy in-tree module ``tools.web_providers.ddgs`` was removed in the
+same commit that moved this code under ``plugins/``; this file is now the
+canonical implementation.
 
 The ``ddgs`` package is an optional dependency. ``is_available()`` reflects
 whether the package is importable; the plugin still registers either way so

plugins/web/searxng/provider.py

@@ -1,8 +1,10 @@
 """SearXNG search — plugin form.
 
 Subclasses :class:`agent.web_search_provider.WebSearchProvider`. Same JSON
-API call (``/search?format=json``), same result normalization as the legacy
-:mod:`tools.web_providers.searxng` module.
+API call (``/search?format=json``), same result normalization. The legacy
+in-tree module ``tools.web_providers.searxng`` was removed in the same
+commit that moved this code under ``plugins/``; this file is now the
+canonical implementation.
 
 Search-only — SearXNG aggregates results from upstream engines but does not
 fetch/extract arbitrary URLs. ``supports_extract()`` returns False.