Add universal cloud cred ref ID and orphaned instance cleanup

Summary

This PR implements a system for cleaning up orphaned cloud instances from validation testing. The changes include:

1. Universal Cloud Credential Reference ID: Added a constant UniversalCloudCredRefID = "brev-validation-test" in the validation package that all validation tests now use
2. Cleanup Function: Created CleanupOrphanedInstances() that identifies and terminates instances older than 1 hour with the universal credential reference ID
3. GitHub Action Workflow: Added a disabled-by-default workflow that can run cleanup hourly (commented out schedule, manual trigger only)
4. CLI Tool: Created cmd/cleanup/main.go with dry-run support for manual/automated cleanup operations

The implementation targets Lambda Labs initially but is designed to be extensible to other cloud providers.

Review & Testing Checklist for Human

• Test dry-run mode: Run go run cmd/cleanup/main.go -provider=lambdalabs -dry-run=true to verify it correctly identifies orphaned instances without deleting them
• Verify time-based filtering: Check that the 1-hour cutoff logic works correctly and handles timezones properly (instances created <1hr ago should NOT be cleaned up)
• Test GitHub Action workflow: Manually trigger the workflow via GitHub Actions UI to ensure it runs correctly and respects the disabled-by-default configuration
• Review credential matching logic: Ensure CloudCredRefID matching only targets instances created with the universal validation credential, not production instances
• End-to-end CLI testing: Test both dry-run and actual cleanup modes with real Lambda Labs credentials to verify the complete flow works

Diagram

%%{ init : { "theme" : "default" }}%%
flowchart TD
    ValidationSuite["internal/validation/suite.go<br/>UniversalCloudCredRefID constant<br/>CleanupOrphanedInstances function"]:::major-edit
    ValidationTest["internal/lambdalabs/v1/validation_test.go<br/>Updated to use universal constant"]:::minor-edit
    GitHubAction[".github/workflows/cleanup-orphaned-instances.yml<br/>Disabled by default workflow"]:::major-edit
    CLITool["cmd/cleanup/main.go<br/>Command-line cleanup tool"]:::major-edit
    LambdaLabsClient["internal/lambdalabs/v1/instance.go<br/>ListInstances & TerminateInstance"]:::context
    
    ValidationSuite --> ValidationTest
    GitHubAction --> CLITool
    CLITool --> ValidationSuite
    ValidationSuite --> LambdaLabsClient
    
    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

Notes

• The GitHub Action is intentionally disabled by default (manual trigger only) as requested - uncomment the schedule section to enable hourly runs
• The cleanup function includes comprehensive error handling and logging for debugging
• The CLI tool supports both Lambda Labs initially but is structured to easily add other cloud providers
• All linting and existing tests pass successfully

Link to Devin run: https://app.devin.ai/sessions/ed3c049e29244357b8ea2587a22d7cd8
Requested by: @theFong