bin/scratch: rename 'mine' subcommand to 'list' and 'who' arg to 'owner'

[aljoscha]

Rename the 'mine' subcommand to 'list' for clarity, since the command lists scratch instances and isn't limited to showing your own. Also rename the positional 'who' argument to 'owner' to better describe its purpose.

Update documentation in doc/developer/scratch.md accordingly.

Note: other subcommands (ssh, sftp, push, forward, destroy) accept the literal string 'mine' as an instance ID shorthand to refer to your only live instance. This is a separate concept from the subcommand name and is intentionally left unchanged.

[github-actions]

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[def-]

Should we maybe keep an alias for the old mine subcommand?

[aljoscha]

Should we maybe keep an alias for the old mine subcommand?

Could, but I feel like a clean cut is better. We don't have automated tooling using this, do we?

[def-]

Nope!