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/ouyi/meta.json

@@ -60,12 +60,31 @@
       "tiantai"
     ],
     "keywords": [
+      "蕅益",
       "天台",
       "净土",
       "念佛",
+      "弥陀要解",
+      "教宗天台",
+      "行归净土",
+      "禅教律净",
+      "融通",
+      "性相融会",
+      "现前一念",
+      "一念心性",
+      "事持",
+      "理持",
+      "持名念佛",
       "六信",
+      "信自",
+      "信他",
+      "信因",
+      "信果",
+      "信事",
+      "信理",
+      "教观纲宗",
       "止观",
-      "性相融会"
+      "明末四大高僧"
     ]
   }
 }