fix(mdm): reject shell metacharacters in --bin paths

The path is interpolated into the launchd `sh -c` command line and the
systemd ExecStart line, so restrict it to a safe character set. --env values
are never shell-evaluated: launchd passes them through EnvironmentVariables
and systemd through Environment=, and parameter expansion results are not
re-parsed by the shell.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>