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/xuyun/meta.json

@@ -49,12 +49,30 @@
       "dingfubao"
     ],
     "keywords": [
+      "虚云",
       "参禅",
       "话头",
+      "参话头",
+      "念佛是谁",
       "疑情",
       "开悟",
+      "桶底脱落",
       "禅七",
-      "禅净双修"
+      "行香",
+      "丛林",
+      "五宗兼嗣",
+      "临济",
+      "曹洞",
+      "沩仰",
+      "云门",
+      "法眼",
+      "老实修行",
+      "头陀行",
+      "持戒",
+      "禅净双修",
+      "云居山",
+      "南华寺",
+      "云门寺"
     ]
   }
 }