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/yinguang/meta.json

@@ -62,12 +62,30 @@
       "dingfubao"
     ],
     "keywords": [
-      "念佛",
       "净土",
-      "往生",
+      "念佛",
+      "持名念佛",
+      "十念法",
+      "摄耳谛听",
+      "老实念佛",
       "信愿行",
+      "带业往生",
+      "仗佛慈力",
+      "自力他力",
+      "竖出横超",
+      "往生",
+      "极乐",
       "阿弥陀",
-      "极乐"
+      "阿弥陀佛",
+      "净土三经",
+      "敦伦尽分",
+      "闲邪存诚",
+      "因果报应",
+      "诸恶莫作",
+      "众善奉行",
+      "文钞",
+      "一函遍复",
+      "印光"
     ]
   }
 }