feat(polymarket): add Polymarket prediction markets as 6th research source

Search Polymarket's free Gamma API for relevant prediction markets on any
topic. Uses smart multi-query expansion to cast a wider net (e.g., "Arizona
Basketball" also searches "Arizona"), merges and dedupes by event ID, and
shows price movement context ("up 22.5% this week"). No API key required.

Also hides sources with zero results from the stats output (all sources).

54 new tests, all passing. Full pipeline integration with scoring, dedupe,
cross-source linking, and rendering.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/lib/env.py

@@ -254,6 +254,14 @@ def is_hackernews_available() -> bool:
     return True
 
 
+def is_polymarket_available() -> bool:
+    """Check if Polymarket source is available.
+
+    Always returns True - Gamma API is free, no key needed.
+    """
+    return True
+
+
 def get_x_source_status(config: Dict[str, Any]) -> Dict[str, Any]:
     """Get detailed X source status for UI decisions.