feat: smart master selection + expanded keywords (6→24 per master)

- /compare-masters now uses keyword matching from meta.json search_scope.keywords
- 3-tier selection: user-specified → keyword matching → topic fallback
- Expanded each master's keywords to 15-25 core concepts (avg 24)
- Document --masters manual override in README

Selection improvements verified:
- '遍行因' → xuanzang (5 matches)
- '明心见性' → huineng (3 matches)
- '空性/中观' → kumarajiva + huineng
- '念佛' → yinguang + ouyi + xuyun

prebuilt/kumarajiva/meta.json

@@ -67,11 +67,29 @@
     ],
     "keywords": [
       "空",
+      "中观",
+      "三论",
       "中道",
       "般若",
-      "法华",
+      "缘起",
+      "性空",
+      "毕竟空",
+      "八不",
       "不二",
-      "八不"
+      "实相",
+      "诸法实相",
+      "法华",
+      "一佛乘",
+      "方便",
+      "会三归一",
+      "维摩诘",
+      "金刚经",
+      "中论",
+      "大智度论",
+      "十二门论",
+      "百论",
+      "佛性",
+      "大乘"
     ]
   }
 }