Add golang program with integer add function

[devin-ai-integration]

Add golang program with integer add function

Summary

This PR adds a new golang program that implements integer addition functionality with both a library API and command-line interface. The implementation includes:

• Add(a, b int) int function for adding two integers
• AddMultiple(numbers ...int) int function for adding multiple integers
• Command-line interface supporting variable number of integer arguments
• Comprehensive unit tests using testify framework
• Proper error handling for invalid inputs

Additionally, this PR fixes a CI issue by updating the deprecated actions/upload-artifact@v2 to v4 in the GitHub Actions workflow.

Review & Testing Checklist for Human

• Repository placement: Verify that adding this golang program to the omil repository (network monitoring tool) is appropriate, or if it should be in a separate repository
• Functional testing: Test the CLI interface manually with various inputs: go run cmd/add/main.go 5 10, go run cmd/add/main.go 1 2 3 4 5, go run cmd/add/main.go -5 10
• Error handling: Test with invalid inputs like go run cmd/add/main.go abc 123 and go run cmd/add/main.go 5 (insufficient args)
• Mathematical correctness: Review the Add and AddMultiple function implementations for edge cases (negative numbers, zero, large numbers)
• CI fix appropriateness: Confirm that updating upload-artifact from v2 to v4 is the correct solution for the deprecated action warning

Test Plan: Navigate to cmd/add/ directory and run go test -v to verify all tests pass, then test the CLI with various integer combinations to ensure proper functionality.

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    A["cmd/add/main.go<br/>New CLI program"]:::major-edit
    B["cmd/add/add_test.go<br/>Unit tests"]:::major-edit
    C["cmd/add/go.mod<br/>Module definition"]:::major-edit
    D["cmd/add/go.sum<br/>Dependencies"]:::major-edit
    E["cmd/add/README.md<br/>Documentation"]:::major-edit
    F[".github/workflows/main.yml<br/>CI workflow"]:::minor-edit
    G["omil repository<br/>Root project"]:::context
    
    A --> B
    A --> C
    C --> D
    G --> A
    G --> F
    
    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit  
        L3[Context/No Edit]:::context
    end

    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• The CI failure in OSS upload is unrelated to these changes - it's a timeout issue with Alibaba Cloud OSS that was pre-existing
• All core CI steps (unit tests, build, artifact upload) are passing successfully
• The testify dependency follows the user's specified preference for Go unit testing
• Link to Devin run: https://app.devin.ai/sessions/49a7971480b14077b924ff706a403630
• Requested by: yangchengzhi2016@gmail.com

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