feat(web_search): add freshness parameter for Brave time filtering

Adds support for Brave Search API's freshness parameter to filter results
by discovery time:
- 'pd' - Past 24 hours
- 'pw' - Past week
- 'pm' - Past month
- 'py' - Past year
- 'YYYY-MM-DDtoYYYY-MM-DD' - Custom date range

Useful for cron jobs and monitoring tasks that need recent results.

Note: Perplexity provider ignores this parameter (Brave only).

---
🤖 AI-assisted: This PR was created with Claude (Opus). Lightly tested via
build script. The change follows existing patterns for optional parameters
(country, search_lang, ui_lang).

src/agents/tools/web-search.ts

@@ -252,7 +252,9 @@ function isValidIsoDate(value: string): boolean {
 
   const date = new Date(Date.UTC(year, month - 1, day));
   return (
-    date.getUTCFullYear() === year && date.getUTCMonth() === month - 1 && date.getUTCDate() === day
+    date.getUTCFullYear() === year &&
+    date.getUTCMonth() === month - 1 &&
+    date.getUTCDate() === day
   );
 }