support a 1-target .ind pattern

The vision: rather than 2-target or 3-target patterns for an indicator file and data file (or temp indicator file, cache indicator file, and data file for 3-target version), offer a 1-target option that looks like this:
```
process/data_stores.rds.ind:
command: processing_function(outind = target_name, inind = 'process/aggregated_months.rds.ind')
cache: TRUE
```
We think this would require some kind of text-processing layer that takes a remake yml and adds the .rds target and command to targets that have a certain argument specified, then runs `scmake` as it currently exists. The wrapper might also push and pull from s3 or drive for you, according to some configuration you set up.

This task will be substantial, and we should have a detailed plan in place before attempting any code changes.

Begin by documenting how existing 2-target and 3-target .ind patterns are represented in remake yml and how scmake currently processes them. Define the proposed 1-target syntax, text-processing layer, and optional S3 or Drive behavior before changing code; done means the project has a detailed, agreed plan for implementation.

Written by the indexing model from the issue text.