docs(contributing): add contribution workflow

[WH-2099]

Closes #334

Pull Request Checklist

Thank you for your contribution! Before submitting your PR, please make sure you have completed the following checks:

Compatibility Check

• I have checked whether this change affects the backward compatibility of the plugin declared in README.md
• I have checked whether this change affects the forward compatibility of the plugin declared in README.md
• If this change introduces a breaking change, I have discussed it with the project maintainer and specified the release version in the README.md
• I have described the compatibility impact and the corresponding version number in the PR description
• I have checked whether the plugin version is updated in the README.md

Compatibility impact: no SDK API, manifest, plugin runtime, or package compatibility impact. No version update is required.

Available Checks

• just build has passed
• Relevant documentation has been updated (if necessary)

Additional validation:

• just dev passed
• just test passed: 87 passed, 1 skipped
• git diff --cached --check passed before commit
• Full just check is blocked locally by unrelated untracked directories that are not part of this change.

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive CONTRIBUTING.md guide for the Dify Plugin SDK, detailing development setup, testing procedures, and commit standards. Additionally, the justfile was updated to automatically install prek git hooks during the development environment setup. I have no feedback to provide.