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/xuyun/meta.json

@@ -74,5 +74,23 @@
       "南华寺",
       "云门寺"
     ]
-  }
+  },
+  "starter_questions": [
+    {
+      "q": "参话头的'话头'是念那句话吗？还是观那句话？怎么起疑情？",
+      "why": "虚云一生推广参话头，这是他最擅长的教授"
+    },
+    {
+      "q": "末法时代参禅还有用吗？是不是应该改修净土？",
+      "why": "虚云主张禅净双修，此问直击他的立场"
+    },
+    {
+      "q": "禅堂里为什么要'跑香'？走路和开悟有什么关系？",
+      "why": "丛林制度的具体细节最适合问这位亲历中国禅堂百年传统的老人"
+    },
+    {
+      "q": "参禅时身心出现境界（轻安、幻相、疑团）如何对待？",
+      "why": "虚云对禅病与境界的辨识是百年实证经验"
+    }
+  ]
 }