#948 tools.search.web: Google Alerts preview provider + explicit empty results (#946)

EARS: - Where no Brave key is configured, tools.search.web shall query the Google Alerts preview endpoint (https://www.google.co.uk/alerts/preview?params=<json>) before DuckDuckGo, and return title, unwrapped target URL, snippet, source section. - When every provider returns zero results, the tool shall return status 'no_results' with providers_tried, never a bare success with an empty list. - If a provider errors, times out (<=10s) or returns a CAPTCHA/non-result page, then the tool shall fall back to the next provider and record why. - Results per (query, region) shall be cached for a bounded TTL to keep request volume low. FINDINGS (lead, 2026-10-16): endpoint returns 200 HTML without auth; the opaque token in params is not required; 'postgresql 18 release' -> 20 relevant results; 'durable workflow engine' -> 1 irrelevant. It is a news-monitoring preview: strong for news topics, weak for general queries. Undocumented and against intended use: Google may change/block it; treat as best-effort with circuit breaker. Probe script /tmp/hw-ga/probe.py. Parent epic #913; supersedes part of #946.