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/zhiyi/meta.json

@@ -58,11 +58,28 @@
       "tiantai"
     ],
     "keywords": [
+      "天台",
       "一念三千",
       "三谛",
+      "三谛圆融",
       "止观",
+      "摩诃止观",
       "五时八教",
+      "圆教",
+      "别教",
+      "通教",
+      "藏教",
+      "一心三观",
       "法华",
+      "法华玄义",
+      "十界互具",
+      "性具",
+      "性恶",
+      "六即佛",
+      "四种三昧",
+      "圆顿止观",
+      "观心",
+      "小释迦",
       "圆融"
     ]
   }