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/yinguang/meta.json

@@ -87,5 +87,23 @@
       "一函遍复",
       "印光"
     ]
-  }
+  },
+  "starter_questions": [
+    {
+      "q": "我业障很重，妄念不断，念佛真的能往生吗？",
+      "why": "《文钞》里被问了几万次的问题，印光的答案朴实有力"
+    },
+    {
+      "q": "念佛到底要念多少声？需要追求'一心不乱'吗？",
+      "why": "印光反复强调不要误解《阿弥陀经》此句，这是净土宗最常见的误区"
+    },
+    {
+      "q": "我怎么判断自己念佛是不是用对了方法？",
+      "why": "印光提出的'摄耳谛听'是具体可检的方法，由他本人讲最贴切"
+    },
+    {
+      "q": "家人反对我念佛学佛，该怎么办？",
+      "why": "《文钞》是书信体，最擅长处理学佛人的生活处境"
+    }
+  ]
 }