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

internal/api/router/router.go

@@ -92,8 +92,6 @@ func WithSkipPaths(paths ...string) MiddlewareOption {
 }
 
 // NewHumaAPI creates a new Huma API with all routes registered
-//
-//nolint:ireturn // huma.API is the expected interface type for Huma APIs
 func NewHumaAPI(cfg *config.Config, registry service.RegistryService, mux *http.ServeMux, metrics *telemetry.Metrics) huma.API {
 	// Create Huma API configuration
 	humaConfig := huma.DefaultConfig("MCP Registry API", "1.0.0")