chore(skills): sync .gemini and .github skills and fix i18n validator

.github/skills/README.md

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+# Agent Skills Index
+
+This folder contains reusable Agent Skills for GitHub Copilot / VS Code custom agent workflows.
+
+## Available Skills
+
+- **community-announcer**
+  - Purpose: Generate community announcement content and related assets.
+  - Entry: `community-announcer/SKILL.md`
+
+- **doc-mirror-sync**
+  - Purpose: Sync mirrored documentation content and helper scripts.
+  - Entry: `doc-mirror-sync/SKILL.md`
+
+- **gh-issue-replier**
+  - Purpose: Draft standardized issue replies with templates.
+  - Entry: `gh-issue-replier/SKILL.md`
+
+- **gh-issue-scheduler**
+  - Purpose: Schedule and discover unanswered issues for follow-up.
+  - Entry: `gh-issue-scheduler/SKILL.md`
+
+- **i18n-validator**
+  - Purpose: Validate translation key consistency across i18n dictionaries.
+  - Entry: `i18n-validator/SKILL.md`
+
+- **plugin-scaffolder**
+  - Purpose: Scaffold OpenWebUI plugin boilerplate with repository standards.
+  - Entry: `plugin-scaffolder/SKILL.md`
+
+- **version-bumper**
+  - Purpose: Assist with semantic version bumping workflows.
+  - Entry: `version-bumper/SKILL.md`
+
+- **xlsx-single-file**
+  - Purpose: Single-file spreadsheet operations workflow without LibreOffice.
+  - Entry: `xlsx-single-file/SKILL.md`
+
+## Notes
+
+- Skill definitions follow the expected location pattern:
+  - `.github/skills/<skill-name>/SKILL.md`
+- Each skill may include optional `assets/`, `references/`, and `scripts/` folders.
+- This directory mirrors `.gemini/skills` for compatibility.