fix(planner): register xquik in SOURCE_CAPABILITIES (#319)

Without this entry, the planner's _default_sources_for_intent() drops
xquik from the candidate pool for how_to / comparison / news intents
because SOURCE_CAPABILITIES.get("xquik", set()) returns the empty set.
Users with XQUIK_API_KEY set get zero Xquik results even though the
engine recognizes the key.

Mirrors the capabilities for "x" since both are X/Twitter-shaped
discussion + social sources.

Fixes #319

skills/last30days/scripts/lib/planner.py

@@ -60,6 +60,7 @@ INTENT_SOURCE_EXCLUSIONS: dict[str, set[str]] = {
 SOURCE_CAPABILITIES = {
     "reddit": {"discussion", "social"},
     "x": {"discussion", "social"},
+    "xquik": {"discussion", "social"},
     "youtube": {"video", "video_longform", "discussion"},
     "tiktok": {"video", "video_shortform", "social"},
     "instagram": {"video", "video_shortform", "social"},