doc: create CONTRIBUTION.md for Claude command-based workflow

[p-iknow]

Context

This project (oss-kit) is designed to be used as a template for general OSS projects. It leverages Claude custom commands to streamline the entire contribution workflow from issue creation to PR submission. Currently, there is no CONTRIBUTION.md to guide contributors through this workflow. The README.md references CONTRIBUTION.md but the file doesn't exist yet, and the project will soon be ready to accept external contributions.

Requirement

Create a comprehensive CONTRIBUTION.md that documents the complete contribution workflow using Claude custom commands:

1. Creating issues with /issue-task
2. Creating branches with /branch
3. Planning implementation with /plan
4. Committing changes with /commit
5. Creating pull requests with /pr

The documentation should be suitable for general OSS projects and provide clear, step-by-step guidance.

Content Outline

• Welcome and Prerequisites (Claude Code setup, gh CLI)
• Complete Contribution Workflow
  • Step 1: Create an Issue (/issue-task)
  • Step 2: Create a Branch (/branch)
  • Step 3: Plan Implementation (/plan)
  • Step 4: Commit Changes (/commit)
  • Step 5: Create Pull Request (/pr)
• Commit Convention (reference to CONVENTIONAL_COMMITS.md)
• Code of Conduct reference
• Getting Help

Verification

• Ensure all custom commands are accurately documented
• Verify workflow steps are in correct order
• Test that examples match actual command behavior
• Check that links to other documents (CONVENTIONAL_COMMITS.md, CODE_OF_CONDUCT.md) are correct
• Verify the workflow is suitable for general OSS projects

Reference

• .claude/commands/issue-task.md
• .claude/commands/branch.md
• .claude/commands/plan.md
• .claude/commands/commit.md
• .claude/commands/pr.md
• docs/CONVENTIONAL_COMMITS.md
• README.md