refactor(web): per-capability backend selection for search/extract split

Introduce the foundation for independently selecting web search and
extract backends — enabling future combinations like SearXNG for
search + Firecrawl for extract.

Architecture:
- tools/web_providers/base.py: WebSearchProvider and WebExtractProvider
  ABCs with normalized result contracts (mirrors CloudBrowserProvider)
- tools/web_tools.py: _get_search_backend() and _get_extract_backend()
  read per-capability config keys, fall through to shared web.backend
- hermes_cli/config.py: web.search_backend and web.extract_backend in
  DEFAULT_CONFIG (empty = inherit from web.backend)

Behavioral change:
- web_search_tool() now dispatches via _get_search_backend()
- web_extract_tool() now dispatches via _get_extract_backend()
- When per-capability keys are empty (default), behavior is identical
  to before — _get_search_backend() falls through to _get_backend()

This is purely structural — no new backends are added. SearXNG and
other search-only/extract-only providers can now be added as simple
drop-in modules in follow-up PRs.

12 new tests, 49 existing tests pass with zero regressions.

Ref: #19198

hermes_cli/config.py

@@ -544,7 +544,13 @@ DEFAULT_CONFIG = {
         # via TERMINAL_LOCAL_PERSISTENT env var.
         "persistent_shell": True,
     },
-    
+
+    "web": {
+        "backend": "",           # shared fallback — applies to both search and extract
+        "search_backend": "",    # per-capability override for web_search (e.g. "searxng")
+        "extract_backend": "",   # per-capability override for web_extract (e.g. "native")
+    },
+
     "browser": {
         "inactivity_timeout": 120,
         "command_timeout": 30,  # Timeout for browser commands in seconds (screenshot, navigate, etc.)