Add repo-standardizer skill: polish any GitHub repo surface (#2715)

* Add repo-standardizer skill: polish any GitHub repo surface

* fix(skill): remove non-string metadata and fix template README refs for vally lint

* docs(skill): drop markdown relative links from README language-switcher example

Vally valid-refs flagged ./README.md and ./README.zh.md as missing file
references; they are generated artifacts, not skill files. Keep the
example as plain text to satisfy the linter.

* fix(template): add missing language switcher to English README template

Align with README.zh.md template and SKILL.md switcher convention:
current language as plain text, other languages as relative links.

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Co-authored-by: programmingWTF <programmingWTF@users.noreply.github.com>

skills/repo-standardizer/templates/CONTRIBUTING.md

@@ -0,0 +1,55 @@
+# Contributing
+
+Thanks for your interest in contributing! Please take a moment to read this
+guide so your contribution goes smoothly.
+
+## Getting started
+
+1. Fork the repository.
+2. Create a feature branch: `git checkout -b feat/your-feature`.
+3. Make your changes and commit them with a clear message.
+4. Push and open a pull request against the default branch (`main` for
+   most repos).
+
+## Pull request checklist
+
+- [ ] The PR description explains what and why.
+- [ ] Tests pass locally (`npm test` / `pytest` / `go test` ...).
+- [ ] New behavior is covered by tests.
+- [ ] Documentation is updated if user-facing behavior changed.
+
+## Issue conventions
+
+- Use the issue forms: bug reports, feature requests, questions.
+- Label your issue with the matching `bug` / `enhancement` / `P0`–`P3` label.
+
+## Commit style
+
+Use conventional commits **with a scope**: `type(scope): description`
+
+```
+feat(labels): add tier labels
+fix(ci): correct release workflow
+```
+
+Common types: `feat` `fix` `docs` `chore` `refactor` `test` `ci` `perf`.
+Scope = the area you touched (module, file, subsystem).
+
+Examples:
+
+```
+feat(labels): add tier labels
+fix(ci): correct release workflow
+fix(docx): update README.md
+docs(readme): explain installation
+chore(ci): bump action version
+```
+
+## Code style
+
+Match the existing style of the project (linter configs are included).
+When in doubt, run the linter before pushing.
+
+## Questions?
+
+Open a discussion or ask in a `question` issue. We're friendly!