feat: add AGENTS.md and vendor-neutral agent skills

.agents/skills/prh-terminology/SKILL.md

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+---
+name: prh-terminology
+description: "Manage the translation terminology dictionary prh.yml for consistent Angular-ja documentation. Use before adopting a translation for a new technical term, when checking existing terminology rules, and when adding, changing, or removing rules in prh.yml."
+---
+
+# prh-terminology: Terminology dictionary management
+
+Maintain the Angular-ja project's translation terminology dictionary in `prh.yml` so that Japanese technical terms stay consistent across all Angular documentation translations.
+
+**Rule**: Never guess terminology. Check `prh.yml` (and apply this skill) before adopting a translation for a new technical term. Typical moments:
+
+- Before any translation work: check the existing rules for the terms in the file.
+- For a new technical term: decide a consistent Japanese form.
+- For katakana standardization: verify long vowel marks and spelling.
+- After translating: add rules so the chosen form is enforced project-wide.
+
+## prh.yml structure overview
+
+The `prh.yml` file uses the prh (Proofreading Helper) format to enforce consistent terminology:
+
+```yaml
+version: 1
+imports:
+  - path: ./node_modules/prh/prh-rules/files/markdown.yml
+rules:
+  - expected: 正しい表記        # Preferred term
+    pattern:                   # Terms to replace
+      - 間違った表記1
+      - 間違った表記2
+    options:                   # Optional settings
+      wordBoundary: true       # Match whole words only
+```
+
+## Rule categories
+
+### 1. 言い換え (Paraphrasing rules)
+
+Standardizes Japanese technical expressions.
+
+- "依存性の注入" (preferred) vs "依存関係の注入" (to replace)
+- "変更検知" (preferred) vs "変更検出" (to replace)
+
+### 2. カタカナ語 (Katakana rules)
+
+Enforces consistent katakana spelling and English preservation.
+
+- **Long vowel consistency**: "サーバー" not "サーバ"
+- **English preservation**: "Promise" not "プロミス"
+- **Regex patterns**: `/pattern(?!suffix)/` to match specific cases
+
+## How to add or modify rules
+
+### Simple paraphrasing rule
+
+```yaml
+- expected: 推奨される表記
+  pattern:
+    - 置き換える表記1
+    - 置き換える表記2
+```
+
+### Katakana consistency rule
+
+```yaml
+- expected: サーバー
+  pattern: /サーバ(?!ー)/    # Matches "サーバ" not followed by "ー"
+```
+
+### English preservation rule
+
+```yaml
+- expected: Promise
+  pattern: プロミス
+```
+
+### Complex regex rule
+
+```yaml
+- expected: ため
+  pattern: /(行|作)?為(?!替)/
+  regexpMustEmpty: $1        # Captured group must be empty
+```
+
+### Word boundary rule
+
+```yaml
+- expected: Service Worker
+  pattern:
+    - サービスワーカー
+  options:
+    wordBoundary: true       # Match whole words only
+```
+
+## Management procedures
+
+### Add a new rule
+
+1. **Identify category**: paraphrasing or katakana.
+2. **Determine the preferred form**: based on project standards.
+3. **List incorrect forms**: common variations to replace.
+4. **Add appropriate options**: `wordBoundary`, regex patterns.
+
+### Modify an existing rule
+
+1. **Locate the existing rule**: search by expected term.
+2. **Update patterns**: add or remove incorrect forms.
+3. **Adjust options**: modify regex or boundary settings.
+4. **Test impact**: consider existing translations.
+
+### Remove a rule
+
+1. **Verify necessity**: confirm the rule is no longer needed.
+2. **Check dependencies**: ensure no conflicts with existing translations.
+3. **Document the reason**: give a clear justification for the removal.
+
+## Angular-specific terminology guidelines
+
+### Technical terms (keep in English)
+
+- Component, Directive, Service, Pipe
+- Promise, Observable, Signal
+- Router, Guard, Resolver
+
+### Japanese translations (standardize)
+
+- "依存性の注入" for Dependency Injection
+- "変更検知" for Change Detection
+- "遅延読み込み" for Lazy Loading
+
+### Katakana consistency
+
+- Long vowel marks: アプリケーション, サーバー, ユーザー
+- Short forms: ブラウザ (not ブラウザー)
+
+## Validation process
+
+After modifying `prh.yml`:
+
+1. **Test with prh**: verify the syntax is valid.
+2. **Run textlint** (`pnpm lint`): check integration with linting.
+3. **Test on sample files**: verify the rules work correctly.
+4. **Document changes**: update the rule rationale.
+
+## Best practices
+
+1. **Consistency first**: align with existing project terminology.
+2. **Community input**: consider Angular-ja community preferences.
+3. **Technical accuracy**: maintain precision in technical terms.
+4. **Readability**: balance consistency with natural Japanese.
+5. **Incremental changes**: add rules gradually to avoid disruption.
+
+Always test rule changes against existing translations to ensure they improve consistency without introducing errors.

.agents/skills/translate-file/SKILL.md

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+---
+name: translate-file
+description: "Translate English Angular documentation file to Japanese following strict project standards. Use when translating new .en.md/.en.ts/.en.json files into Japanese with line-count preservation, anchor IDs, and terminology consistency."
+---
+
+Translate the file the user specifies (a path under `adev-ja/`) from English to Japanese following the angular-ja project's strict translation standards.
+
+You are an expert Japanese technical translator specializing in Angular documentation translation. Follow the same approach as the project's AI translation tool with two-stage processing: translation → proofreading.
+
+## Critical Translation Rules
+
+**NEVER wrap the entire text in code blocks.** Always maintain original formatting.
+
+**Structural Requirements:**
+- **Maintain EXACT line count** - input and output must have identical number of lines
+- **Preserve markdown structure absolutely** - never change heading levels, list markers, or indentation
+- **Keep empty lines as empty lines** - preserve all whitespace and spacing
+- **Maintain list hierarchy and markers** (*, -, +, 1., etc.)
+- **Preserve code blocks unchanged** - never translate content inside code blocks
+- **Keep URLs, filenames, and identifiers untranslated**
+- **Preserve HTML tags and special symbols exactly**
+
+**AI Translation tool**
+- For Markdown files, use the translator tool: `tools/translator`.
+
+**Heading ID Rules:**
+- For headings **below h1 level** (`<h2>` and lower), add anchor IDs based on original English heading
+- Convert original heading to lowercase and remove all symbols except hyphens
+- Format: `## Japanese Translation {#original-heading-lowercase}`
+- Examples:
+  - `## How to use Angular` → `## Angularの使い方 {#how-to-use-angular}`
+  - `### `foo.bar`` → `### `foo.bar` {#foobar}` (remove all non-hyphen symbols)
+- **h1 level headings get NO anchor IDs**
+
+**Special Content Rules:**
+- **Never change special prefixes**: NOTE/TIP/HELPFUL/IMPORTANT/QUESTION/TLDR/CRITICAL remain in English
+  - Correct: `NOTE: これは重要な情報です。`
+  - Incorrect: `注: これは重要な情報です。`
+- **No spaces around English words**: `Angularの使い方` not `Angular の使い方`
+- **Preserve all Angular terminology**: component, directive, service, pipe, etc.
+
+**Quality Standards:**
+- Follow textlint rules for Japanese technical writing
+- Use appropriate katakana for foreign terms per Ministry of Education guidelines
+- Maintain consistency with existing project translations
+- Ensure all cross-references and internal links remain functional
+
+**CRITICAL: Use the `prh-terminology` skill for Terminology**
+- **BEFORE translating**: Follow the `prh-terminology` skill to check existing terminology rules in `prh.yml`
+- **When encountering new terms**: Follow the same skill for a consistent translation approach
+- **For uncertain katakana**: Check long vowel marks and standardization with the same skill
+- **After translation**: Add new terminology rules per the same skill if needed
+- The skill ensures all translations follow the project's terminology dictionary
+
+**File Management Rules:**
+- **Original content preservation**: Translated Japanese file has ALWAYS corresponding `<name>.en.<ext>` file to preserve original content snapshot
+- **Structure matching**: Translated file MUST have the same line count and document structure to the original file
+- **Diff compatibility**: This enables easy comparison and tracking of changes between versions
+
+**Processing Approach:**
+1. **Block-based translation**: Process content in heading-based blocks like the AI tool
+2. **Two-stage validation**: Translate first, then apply textlint-based corrections
+3. **Preserve technical accuracy** while making content accessible to Japanese developers
+4. **Maintain line correspondence** for easy diff tracking between .en.md and .md files
+
+Always translate only the requested content, returning the translated text without additional explanations or wrapper text.