How do I list all context namespaces in Oracle DB?

You can query the DBA_CONTEXT (or [ALL_CONTEXT][1]) view depending on your privileges and what contexts you're looking at. ALL_CONTEXT will list all the contexts that have attributes set in the current session. DBA_CONTEXT lists all the contexts in the database. However, you need to have additional privileges to be able to query the DBA_CONTEXT view (the SELECT ANY DICTIONARY privilege or SELECT_CATALOG_ROLE role would be more than sufficient but you can also be granted access to that view specifically)

SELECT namespace,
       schema,
       package,
       type
  FROM dba_context

will have a row for the HR_SECURITY namespace showing that it is associated with the PKG_SECURITY package with a TYPE of ACCESSED GLOBALLY.