syz-cluster: commands to triage reports

[a-nogikh]

We need to support at least:

• #syz invalid to close false positives
• #syz unrelated or #syz ignore to indicate that the finding will be addressed separately (so that syzbot doesn't re-report it every time the series is re-submitted).

There happen to be cases when the patch series has helped uncover an existing bug rather than introduced it, see e.g.

• https://lore.kernel.org/all/aKWytdZ8mRegBE0H@mail.gmail.com/
• https://lore.kernel.org/all/aJxihWEdn5cVlP6z@strlen.de/

[a-nogikh]

Initial support for #syz invalid: #6483