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/huineng/meta.json

@@ -46,12 +46,28 @@
     ],
     "keywords": [
       "自性",
-      "顿悟",
-      "见性",
-      "无念",
-      "禅",
       "本心",
-      "般若"
+      "本性",
+      "佛性",
+      "顿悟",
+      "顿教",
+      "南宗禅",
+      "见性",
+      "明心见性",
+      "直指人心",
+      "不立文字",
+      "无念",
+      "无住",
+      "无相",
+      "禅",
+      "六祖",
+      "坛经",
+      "本来无一物",
+      "般若",
+      "定慧等学",
+      "机锋",
+      "菩提",
+      "弘忍"
     ]
   }
 }