Add stylecheck linter, replacement for golint

This revealed 2 capitalized error messages.

https://golangci-lint.run/usage/linters/#stylecheck

.golangci.yml

@@ -49,7 +49,7 @@ linters:
     #- scopelint
     #- staticcheck
     #- structcheck ! deprecated since v1.49.0; replaced by 'unused'
-    #- stylecheck
+    - stylecheck
     #- typecheck
     - unconvert
     #- unparam

command.go

@@ -875,7 +875,7 @@ func (c *Command) ArgsLenAtDash() int {
 
 func (c *Command) execute(a []string) (err error) {
 	if c == nil {
-		return fmt.Errorf("Called Execute() on a nil Command")
+		return fmt.Errorf("called Execute() on a nil Command")
 	}
 
 	if len(c.Deprecated) > 0 {

completions.go

@@ -298,7 +298,7 @@ func (c *Command) getCompletions(args []string) (*Command, []string, ShellCompDi
 	}
 	if err != nil {
 		// Unable to find the real command. E.g., <program> someInvalidCmd <TAB>
-		return c, []string{}, ShellCompDirectiveDefault, fmt.Errorf("Unable to find a command for arguments: %v", trimmedArgs)
+		return c, []string{}, ShellCompDirectiveDefault, fmt.Errorf("unable to find a command for arguments: %v", trimmedArgs)
 	}
 	finalCmd.ctx = c.ctx