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/xuanzang/meta.json

@@ -64,11 +64,29 @@
     ],
     "keywords": [
       "唯识",
+      "法相",
       "阿赖耶识",
+      "末那识",
       "三性",
+      "遍计所执",
+      "依他起",
+      "圆成实",
       "五位百法",
       "因明",
-      "瑜伽"
+      "瑜伽",
+      "种子",
+      "熏习",
+      "转识成智",
+      "六因",
+      "遍行因",
+      "俱有因",
+      "异熟因",
+      "能作因",
+      "四缘",
+      "五种不翻",
+      "成唯识论",
+      "瑜伽师地论",
+      "唯识三十颂"
     ]
   }
 }