Support for shell init files

We use SSM sessions extensively for servers which have restricted network environments. One common source of confusion is that the sessions started via SSM through the AWS console do not load common files like `/etc/environment` (presumably because it's not a full PAM session hitting `pam_env`) or the shell config files such as `/etc/profile`.

In my case, it would be especially useful to get `/etc/environment` since that's what our servers have configured for values expected by cron tasks, applications like Apache, etc.

Start by examining how SSM sessions launched through the AWS console initialize their environment, using /etc/environment and /etc/profile as the requested reference files. Compare the current session behavior with the expected shell and environment initialization, then verify that the selected files are loaded consistently without disrupting existing sessions.

Written by the indexing model from the issue text.