docs: create comprehensive CONTRIBUTING.md guide (#5)

[addidea]

Description

Complete development and contribution guide for PicoClaw with detailed setup instructions, code style guidelines, and PR process.

Closes #5

What's Included

✅ Development Environment Setup

• Prerequisites (Go 1.24+, Git, Make)
• Installation steps with make deps
• Dependency management
• Verification commands

✅ Building and Running

• Quick start: make build
• Run from source (no build)
• Multi-platform builds (AMD64, ARM64, RISC-V, Darwin, Windows)
• System-wide installation options

✅ Testing Guide

• Run all tests: make test
• Package-specific tests
• Coverage reports (go test -coverprofile)
• Race detection (go test -race)
• Benchmarks

✅ Code Style Guide

• Go conventions (gofmt, Effective Go)
• Linting: make lint, make lint-fix
• Project structure explanation
• Naming conventions (packages, files, types, functions)
• Error handling patterns
• Testing best practices (table-driven, >80% coverage)

✅ Pull Request Process (6-step guide)

1. Fork and clone
2. Create descriptive branch (feat/fix/docs/test)
3. Make changes with tests
4. Test locally (lint + test + build)
5. Commit with Conventional Commits
6. Push and open PR with checklist

✅ Project Structure
Complete directory tree with explanations:

• cmd/ - Main entry point
• pkg/ - Public packages (agent, bus, channels, providers, tools)
• skills/ - Agent skills
• assets/ - Images, assets

✅ Common Tasks
How-to guides for:

• Add new channel
• Add new tool
• Add new skill
• Update dependencies
• Debug configuration

✅ Community & Support

• Bug reporting template
• Discord, GitHub Discussions links
• License information

Requirements Met

Requirement	Status
Dev environment setup (Go 1.22+, tools)	✅ Go 1.24+
Build and run locally	✅ make build, go run
Run tests	✅ make test, coverage
Code style guide	✅ gofmt, conventions
PR submission process	✅ 6-step guide

Example: PR Checklist

- [ ] Code compiles without warnings
- [ ] All tests pass (`make test`)
- [ ] Linter passes (`make lint`)
- [ ] Documentation updated (if needed)
- [ ] Commit messages follow convention
- [ ] Branch is up to date with `main`

Example: Code Style

// ✅ Good error handling
if err != nil {
    return fmt.Errorf("failed to publish: %w", err)
}

// ✅ Table-driven tests
func TestMessageBus(t *testing.T) {
    tests := []struct {
        name string
        want int
    }{
        {"case 1", 1},
        {"case 2", 2},
    }
    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            // Test logic
        })
    }
}

Benefits

• 📚 Comprehensive guide for new contributors
• 🛠️ Step-by-step setup instructions
• 🧪 Testing best practices
• 📝 Code style standards
• 🔄 Clear PR process
• 🏗️ Project structure overview
• 🔧 Common task guides

Production-ready contribution guide! 📚