Disable ireturn linter rule (#307)

## Summary

- Disable the `ireturn` linter rule in golangci-lint configuration
- Remove all related `nolint:ireturn` comments from the codebase

The `ireturn` rule was flagging factory functions that intentionally
return interfaces for dependency injection, which is a common and valid
Go pattern. Since this pattern is used extensively throughout the
codebase for good architectural reasons, it makes sense to disable this
rule entirely.

It also seems buggy, and seems to conflict with other lint rules -
causing unnecessary toil.See:
https://stackoverflow.com/questions/72385337/golang-linter-always-complains

tools/publisher/auth/github-oidc.go

@@ -15,8 +15,6 @@ type GitHubOIDCProvider struct {
 }
 
 // NewGitHubOIDCProvider creates a new GitHub OIDC provider
-//
-//nolint:ireturn // Factory function intentionally returns interface for dependency injection
 func NewGitHubOIDCProvider(registryURL string) Provider {
 	return &GitHubOIDCProvider{
 		registryURL: registryURL,