@ sign is used for tasks and boards

Hi,

I think it is kind of confusing to reference tasks via `@` sign, because it is also used for boards.

What do you think of using `#` to reference tasks?

instead of:
```
tb --move @13 current
```

we could use:
```
tb --move #13 @current
```

Since the output uses the `@` sign for boards, it should be clearer for the user to reference tasks via `#`. He should be able to distinct between either types via the sign.

Thank you very much for this interesting project 👍

No files or tests are named. Start by locating the CLI parsing and task/board reference handling, then check how the current @ syntax is documented and tested. Done should establish whether # references tasks and @ references boards consistently, including the proposed command examples and any compatibility expectations.

Written by the indexing model from the issue text.