feat: Better table formatting for `shpool list`

[injust]

The shpool list tab-delimited output can look very jumbled if session names differ in length:

NAME    STARTED_AT      STATUS
aaa-bbb-ccccccccc-use2-dddd     2026-01-04T02:45:52.020+00:00   disconnected
eeeeeeee        2025-12-31T18:17:22.541+00:00   disconnected
aaa-bbb-ccccccccc-use2  2026-01-01T21:45:28.675+00:00   disconnected
aaa-bbb-ccccccccc-apne2 2026-01-04T02:13:20.564+00:00   disconnected
aaa-bbb-ccccccccc-apne2-dddd    2026-01-04T02:47:28.542+00:00   disconnected
aaa-bbb-ccccccccc-ilc1  2026-01-01T21:50:18.734+00:00   disconnected
ffff    2026-01-04T04:07:15.237+00:00   disconnected

When outputting to a TTY, it would be more readable if the columns were aligned like a table.

[ethanpailes]

That would break backwards compatibility for anyone parsing the output as TSV. If you want to add a -t/--tabular flag, I would accept a patch for that though.

[injust]

That would break backwards compatibility for anyone parsing the output as TSV.

When the output isn't going to a TTY, we could keep the current TSV output. That should retain backward compatibility.

[ethanpailes]

I'm not the biggest fan of isatty() detection for something like this. It is easier to write scripts against if the output is consistant.

[injust]

Makes sense.

I still think the default output format should be human-first, since it's much easier to add flags in a script. A -t/--tabular flag is a bit inconvenient to use.

Would you consider doing this as a breaking change in the future?

[ethanpailes]

I think it would be reasonable to do as a breaking change when we go to 1.0

[noperator]

In the meantime, consider using column -t to pretty-print the table values as demonstrated here: #292 (comment)