Add systemd validation check to ValidateInstanceImage

[devin-ai-integration]

Add systemd validation check to ValidateInstanceImage

Summary

This PR adds a new validation check to ensure systemd is running on instances, addressing a requirement to validate that systemd is accessible via the systemctl command. The implementation includes:

• New systemd validation: Added Check 4 to ValidateInstanceImage that runs systemctl is-system-running and accepts both "running" and "degraded" states as valid
• Function refactoring: Extracted validation logic into smaller helper functions (connectToInstance, validateArchitecture, validateOSVersion, validateHomeDirectory, validateSystemd) to fix lint issues with function length
• Documentation updates: Updated docs/SECURITY.md and README.md to document the new systemd requirement

The systemd check follows the same error handling pattern as existing validation checks and integrates seamlessly with the current validation pipeline.

Review & Testing Checklist for Human

• Verify systemd state logic: Test that accepting "degraded" systemd state is appropriate for the use case (degraded means some services failed but systemd is still functional)
• Test refactored validation logic: Ensure the helper function extraction didn't introduce bugs in error handling or validation flow
• Test systemd check on target systems: Verify systemctl is-system-running behaves correctly on Ubuntu 20.04/22.04 instances
• Review documentation completeness: Check if other documentation locations should mention the systemd requirement
• End-to-end validation testing: Run the full validation pipeline on real instances to ensure all checks work together correctly

---

Diagram

%%{ init : { "theme" : "default" }}%%
flowchart TD
    subgraph "pkg/v1/image.go"
        A["ValidateInstanceImage()"]:::major-edit
        B["connectToInstance()"]:::major-edit
        C["validateArchitecture()"]:::major-edit
        D["validateOSVersion()"]:::major-edit
        E["validateHomeDirectory()"]:::major-edit
        F["validateSystemd()"]:::major-edit
    end
    
    subgraph "Documentation"
        G["docs/SECURITY.md"]:::minor-edit
        H["README.md"]:::minor-edit
    end
    
    subgraph "Testing Integration"
        I["internal/validation/suite.go"]:::context
    end
    
    A --> B
    A --> C
    A --> D
    A --> E
    A --> F
    I --> A
    
    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:#ADD8E6
    classDef context fill:#FFFFFF

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Notes

• The systemd validation uses systemctl is-system-running which can return various states; this implementation accepts "running" and "degraded" as valid
• The refactoring was necessary to fix lint error about function length (45 > 40 statements)
• Validation tests in internal/validation/suite.go will automatically include the new systemd check since they call ValidateInstanceImage directly
• All changes follow existing patterns in the codebase for error handling and validation logic

Session Details:

• Link to Devin run: https://app.devin.ai/sessions/6962bf2651e6434080cf1459977082f7
• Requested by: @theFong

[devin-ai-integration]

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