fix: --plain flag should imply --table in sprint list

When using `jira sprint list --plain`, the --plain flag was ignored
because only --table was checked in the TUI vs table mode decision.
Adding plain to the condition so --plain bypasses the TUI explorer
and renders a plain table, matching user expectations.

Closes #968

Author: Yoan Wainmann

internal/cmd/sprint/list/list.go

@@ -258,7 +258,7 @@ func sprintExplorerView(sprintQuery *query.Sprint, flags query.FlagParser, board
 	table, err := flags.GetBool("table")
 	cmdutil.ExitIfError(err)
 
-	if table || tui.IsDumbTerminal() || tui.IsNotTTY() {
+	if table || plain || tui.IsDumbTerminal() || tui.IsNotTTY() {
 		cmdutil.ExitIfError(v.RenderInTable())
 	} else {
 		cmdutil.ExitIfError(v.Render())