add an option to `ent list` to show only filtered issue ids

This would be useful in scripts, like:
```
for ISSUE_ID in $(ent list --ids-only ${MY_FILTER}); do
    # do thing with ${ISSUE_ID}
done
```

For now you can sort of emulate this with:
```
ent list state=done done-time=2026-01-01T00:00:00-06:00.. | grep '  ' | cut -f 1 -d ' '
```