feat(web): extend ABC with supports_crawl and async-extract semantics

Two ABC additions to cover the surface area of the remaining four
providers (exa, parallel, tavily, firecrawl) which were untouched by the
initial spike:

1. supports_crawl() + crawl() — Tavily natively crawls a seed URL via
   its /crawl endpoint. Exposing supports_crawl=True lets the crawl
   tool's dispatcher route to Tavily when configured, falling back to
   the auxiliary-model summarization path otherwise. Firecrawl could
   add this in a follow-up (the SDK supports it; we just don't surface
   it as a tool today).

2. Async-or-sync extract() — Parallel's SDK is natively async
   (AsyncParallel.beta.extract); Exa and Tavily are sync; Firecrawl is
   sync but called inside asyncio.to_thread() with a 60s timeout. The
   ABC docstring now permits either shape: implementations declare
   their own sync/async signature and the dispatcher uses
   inspect.iscoroutinefunction to detect and await.

Also adds get_active_crawl_provider() to web_search_registry mirroring
the search/extract resolvers, with web.crawl_backend as the explicit
override config key.

No behavior change on its own — these are scaffolds for the four
remaining provider migrations.

agent/web_search_registry.py

@@ -114,7 +114,7 @@ _LEGACY_PREFERENCE = ("brave-free", "firecrawl", "searxng", "ddgs")
 
 
 def _resolve(configured: Optional[str], *, capability: str) -> Optional[WebSearchProvider]:
-    """Resolve the active provider for a capability ("search" | "extract").
+    """Resolve the active provider for a capability ("search" | "extract" | "crawl").
 
     Resolution rules (in order):
 
@@ -147,6 +147,8 @@ def _resolve(configured: Optional[str], *, capability: str) -> Optional[WebSearc
             return bool(p.supports_search())
         if capability == "extract":
             return bool(p.supports_extract())
+        if capability == "crawl":
+            return bool(p.supports_crawl())
         return False
 
     def _is_available_safe(p: WebSearchProvider) -> bool:
@@ -218,6 +220,20 @@ def get_active_extract_provider() -> Optional[WebSearchProvider]:
     return _resolve(explicit, capability="extract")
 
 
+def get_active_crawl_provider() -> Optional[WebSearchProvider]:
+    """Resolve the currently-active web crawl provider.
+
+    Reads ``web.crawl_backend`` (preferred) or ``web.backend`` (shared
+    fallback) from config.yaml; falls back per the module docstring.
+
+    Crawl is a niche capability — only Tavily implements it among built-in
+    providers. Most callers should expect ``None`` and fall back to a
+    different strategy (e.g. summarize-via-LLM).
+    """
+    explicit = _read_config_key("web", "crawl_backend") or _read_config_key("web", "backend")
+    return _resolve(explicit, capability="crawl")
+
+
 def _reset_for_tests() -> None:
     """Clear the registry. **Test-only.**"""
     with _lock: