chore: add quality tooling (editorconfig, prettier, markdownlint)

.editorconfig

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+# EditorConfig — https://editorconfig.org
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.md]
+trim_trailing_whitespace = false
+
+[Makefile]
+indent_style = tab

.markdownlint.yml

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+# Markdownlint config
+# CJK-aware punctuation support for bilingual content
+
+default: true
+
+MD013:
+  line_length: 200
+  heading_line_length: 120
+  code_block_line_length: 200
+
+MD033: false
+MD034: false
+MD036: false
+
+MD024:
+  siblings_only: true
+
+MD026:
+  punctuation: ".,;:!。，；：！"

.prettierignore

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+node_modules/
+dist/
+_site/
+.wrangler/
+*.min.js
+deno.lock

.prettierrc

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+{
+  "useTabs": false,
+  "tabWidth": 2,
+  "singleQuote": true,
+  "trailingComma": "none",
+  "printWidth": 100,
+  "semi": true,
+  "endOfLine": "lf"
+}