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/dns.go

@@ -5,8 +5,6 @@ type DNSProvider struct {
 }
 
 // NewDNSProvider creates a new DNS-based auth provider
-//
-//nolint:ireturn // Factory function intentionally returns interface for dependency injection
 func NewDNSProvider(registryURL, domain, hexSeed string) Provider {
 	return &DNSProvider{
 		CryptoProvider: &CryptoProvider{

tools/publisher/auth/github-at.go

@@ -63,8 +63,6 @@ type ServerHealthResponse struct {
 }
 
 // NewGitHubATProvider creates a new GitHub OAuth provider
-//
-//nolint:ireturn // Factory function intentionally returns interface for dependency injection
 func NewGitHubATProvider(forceLogin bool, registryURL string) Provider {
 	return &GitHubATProvider{
 		forceLogin:  forceLogin,

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,

tools/publisher/auth/http.go

@@ -5,7 +5,6 @@ type HTTPProvider struct {
 }
 
 // NewHTTPProvider creates a new HTTP-based auth provider
-//nolint:ireturn // Factory function returns interface by design
 func NewHTTPProvider(registryURL, domain, hexSeed string) Provider {
 	return &HTTPProvider{
 		CryptoProvider: &CryptoProvider{

tools/publisher/auth/none.go

@@ -19,7 +19,6 @@ type TokenResponse struct {
 	ExpiresAt     int64  `json:"expires_at"`
 }
 
-//nolint:ireturn // Factory function returns interface by design
 func NewNoneProvider(registryURL string) Provider {
 	return &NoneProvider{
 		registryURL: registryURL,