[Umbrella]: Collection-wide variable naming and specification hygiene

Problem

Variable naming and the role argument specifications are handled ad hoc. Names follow no
documented schema, internal runtime variables are indistinguishable from user-facing ones and
are partly declared in defaults/main.yml and meta/argument_specs.yml, and single-role
variables sit in the elasticstack meta role. As a result every issue that renames something
re-decides the same rules.

Want

One documented naming schema, applied across all roles, with internal variables kept out of the
user-facing contract.

Children

• #542 - defines the rules the others use to pick target names
• #495 - blocked by #542
• #474 - blocked by #495
• #485 - blocked by #542
• #544 - blocked by #495 and #485

#474 has to run after #495, because #495 removes internal variables from defaults/main.yml and
meta/argument_specs.yml - auditing first would cover entries that #495 then deletes. #544
comes last so that no variable is renamed twice.

elasticstack_password needs both an internal prefix and a meaningful name. It is touched only
once, in #495, with the target name __elasticstack_elastic_password.

Begin with child issue #542, which defines the naming rules, then inspect defaults/main.yml and meta/argument_specs.yml across the roles. The umbrella work is done when the documented schema is applied across all roles, internal variables are excluded from the user-facing contract, and the listed child issues are completed in their stated order.