feat(onboarding): browser-first README + starter_questions for 8 masters

Shift the README narrative so that non-developer users — the 99% of the
actual audience — find the browser entry point first and the CLI install
second. The old structure buried fojin.app/chat at line 48 under a wall
of developer badges.

Changes:

- README.md / README_EN.md: new "立即体验 / Try It Now" hero section
  right after the title block, with a big call-to-action linking to
  fojin.app/chat, a "which master should I ask" recommendation table,
  and an explicit redirect for CLI users to the developer section.
- Rename "快速开始 / Quick Start" → "开发者安装 / Developer Installation"
  with a clear "if you just want to try it, use the browser instead"
  callout at the top.
- Update TOC anchors in both languages to match.

- prebuilt/<master>/meta.json × 8: new `starter_questions` field with
  4 Q&A entries per master. Each entry has `q` (the question) and `why`
  (why this question is suited for this specific master's voice and
  source corpus). Intended consumer: fojin.app/chat can fetch the raw
  meta.json on master selection and render these as cold-start prompts
  — solving the empty-input-box problem for first-time users.

Rationale: the biggest onboarding gap for non-CLI users is "I don't
know what to ask this master." A curated 4-question starter per master
provides both a safe entry point and a teaching signal about what this
particular master is actually good at answering.

Validated: scripts/validate.py --strict passes (9/9 masters).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

prebuilt/kumarajiva/meta.json

@@ -91,5 +91,23 @@
       "佛性",
       "大乘"
     ]
-  }
+  },
+  "starter_questions": [
+    {
+      "q": "'空'到底是在否定什么？是不是什么都没有？",
+      "why": "直接问中观最易被误解的核心，由《中论》译者回答最权威"
+    },
+    {
+      "q": "为什么龙树说'因缘所生法，我说即是空'？缘起和性空是同一件事吗？",
+      "why": "《中论》开篇根本偈，罗什译本是中文世界的标准"
+    },
+    {
+      "q": "'应无所住而生其心'——《金刚经》这句话该怎么理解？",
+      "why": "《金刚经》正是罗什译本，由译者本人解读最贴合"
+    },
+    {
+      "q": "中观所说的'二谛'（世俗谛 / 胜义谛）如何区分？学佛人如何在两谛之间行走？",
+      "why": "二谛是中观学派理解世界的基本坐标，直接源自罗什所传龙树学"
+    }
+  ]
 }