Unable to add tag named "d" to a time interval.

When I use `timew tag @18 d`, it should do the following:
`Note: 'd' is a new tag.`
`Added d to @18`

Instead it shows this:
`At least one tag must be specified. See 'timew help tag'.`

When I use `timew tag @18 di`, it works as it is supposed to.

Start by reproducing `timew tag @18 d` and compare it with `timew tag @18 di`. Then trace the `timew tag` argument parsing and its help entry point to find why a one-character tag is rejected. Done means `d` is accepted and produces the reported note and confirmation output without regressing longer tags.

Written by the indexing model from the issue text.