Fix SonarQube High severity issues: string duplication and cognitive complexity

[devin-ai-integration]

Summary

This PR addresses two High severity SonarQube issues in pkg/github/repositories.go:

1. String Literal Duplication (go:S1192): Fixed duplicate string literal "failed to get GitHub client: %w" that appeared 12 times across the file
2. Cognitive Complexity (go:S3776): Reduced cognitive complexity of the ListCommits function from 23 to below the threshold by extracting handler logic

Changes

String Duplication Fix

• Added constant errFailedToGetGitHubClient at package level
• Replaced all 12 occurrences of the duplicated error message string with the constant
• Affects functions: GetCommit, ListCommits, ListBranches, CreateOrUpdateFile, CreateRepository, GetFileContents, ForkRepository, DeleteFile, CreateBranch, PushFiles, ListTags, GetTag

Cognitive Complexity Fix

• Extracted anonymous handler function from ListCommits into standalone listCommitsHandler helper function
• ListCommits now delegates to listCommitsHandler, reducing nesting and complexity
• No functional changes - behavior is identical

Testing

• ✅ All existing unit tests pass
• ✅ Linter passes with 0 issues

Review Checklist

• Verify all 12 string literal replacements are correct and no occurrences were missed
• Confirm listCommitsHandler maintains exact same behavior as original inline handler
• Check if handler extraction pattern is consistent with codebase conventions
• Consider if similar refactoring should be applied to other high-complexity functions for consistency

Notes

• This addresses 1 issue from each SonarQube category as requested
• Many other similar issues remain in the codebase that could benefit from the same treatment
• SonarQube scan should be re-run to confirm these specific issues are resolved

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Link to Devin run: https://app.devin.ai/sessions/7404ee489c2448e0a734739b9c018809
Requested by: Shawn Azman (shawn@cognition.ai) / @ShawnAzman

[devin-ai-integration]

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