docs: add docs-en-improvement skill and remove old prompt file

.agents/skills/docs-en-improvement/SKILL.md

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+---
+name: docs-en-improvement
+description: Improve English documentation under `website/docs/en` by rewriting unnatural translated sentences into clear, professional English while preserving meaning. Use when editing or polishing English docs.
+---
+
+# Docs En Improvement
+
+## Steps
+
+1. Focus on files in `website/docs/en`.
+
+2. Rewrite only sentences that clearly improve clarity, correctness, or naturalness in English.
+
+3. Preserve original meaning exactly; do not introduce new claims or remove technical details.
+
+4. If a file in `website/docs/en` is updated, check the mirrored file in `website/docs/zh` and apply equivalent updates when needed.
+
+## Constraints
+
+- Limit each PR to 10 documentation files or fewer.
+- Start PR titles with `docs:`.
+- Keep reasonable technical abbreviations (for example, `dev server`).
+- Keep wording simple and readable for non-native English readers.
+- Use sentence case for Markdown headings.