ds006688 superseded by ds007116 — swap the stale ID in the curated list + re-run

**TL;DR:** ds006688 is skipped *before* any job: `select-eligible-sub-ses.py` fetches each dataset's sub/ses index from its `OpenNeuroStudies/study-` repo, and **`OpenNeuroStudies/study-ds006688` doesn't exist** → the TSV fetch 404s → selection returns `exit 1` → skipped. Not our `master`-vs-`main` bug (the repo is absent on both); the study repo just isn't there. Sibling to #43 — a recent dataset not fully propagated through the OpenNeuro ecosystem.

## Reproducer

```
python3 select-eligible-sub-ses.py --openneuro-id ds006688 --pipeline fmriprep --count 1 --output /tmp/x.csv
# Fetching .../study-ds006688/master/sourcedata/sourcedata+subjects+sessions.tsv -> not found
# Fetching .../study-ds006688/master/sourcedata/sourcedata+subjects.tsv
# Error fetching TSV: HTTP Error 404: Not Found (exit 1)
gh api repos/OpenNeuroStudies/study-ds006688 # 404 Not Found (study-ds000030 exists)
```

## Action
Wait for / request OpenNeuroStudies to index ds006688; revisit then. Consider a "skip + flag datasets not yet in OpenNeuroStudies" guard so this reports cleanly instead of as a generic selection error.

## Done when
`OpenNeuroStudies/study-ds006688` exists and selection succeeds (or we accept it as not-yet-available).