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/fazang/meta.json

@@ -56,12 +56,28 @@
       "dingfubao"
     ],
     "keywords": [
-      "法界",
       "华严",
-      "十玄",
-      "六相",
+      "华严宗",
+      "法界",
+      "法界缘起",
+      "四法界",
+      "事法界",
+      "理法界",
+      "理事无碍",
       "事事无碍",
-      "因陀罗网"
+      "一即一切",
+      "相即相入",
+      "十玄",
+      "十玄门",
+      "六相",
+      "六相圆融",
+      "圆教",
+      "五教",
+      "金师子章",
+      "因陀罗网",
+      "一真法界",
+      "毗卢遮那",
+      "华严经"
     ]
   }
 }