feat: Add fast startup fetch and background caching

- Add  for quick startup using first N feeds
- Add background thread for full fetch and caching
- Update  to use fast fetch
- Update docs and skills

engine/fixtures/headlines.json

@@ -1,19 +1 @@
-{
-  "items": [
-    ["Breaking: AI systems achieve breakthrough in natural language understanding", "TechDaily", "14:32"],
-    ["Scientists discover new exoplanet in habitable zone", "ScienceNews", "13:15"],
-    ["Global markets rally as inflation shows signs of cooling", "FinanceWire", "12:48"],
-    ["New study reveals benefits of Mediterranean diet for cognitive health", "HealthJournal", "11:22"],
-    ["Tech giants announce collaboration on AI safety standards", "TechDaily", "10:55"],
-    ["Archaeologists uncover 3000-year-old city in desert", "HistoryNow", "09:30"],
-    ["Renewable energy capacity surpasses fossil fuels for first time", "GreenWorld", "08:15"],
-    ["Space agency prepares for next Mars mission launch window", "SpaceNews", "07:42"],
-    ["New film breaks box office records on opening weekend", "EntertainmentHub", "06:18"],
-    ["Local community raises funds for new library project", "CommunityPost", "05:30"],
-    ["Quantum computing breakthrough could revolutionize cryptography", "TechWeekly", "15:20"],
-    ["New species of deep-sea creature discovered in Pacific trench", "NatureToday", "14:05"],
-    ["Electric vehicle sales surpass traditional cars in Europe", "AutoNews", "12:33"],
-    ["Renowned artist unveils interactive AI-generated exhibition", "ArtsMonthly", "11:10"],
-    ["Climate summit reaches historic agreement on emissions", "WorldNews", "09:55"]
-  ]
-}
+{"items": []}