There's a fly in the ointment: we want to store tags as files in a
directory, but file names are not allowed to contain the character `/`,
and some of our current tags use `/` in the tag name.

We could allow subdirectories of the `tags` directory, but it feels
like that breaks the budding database abstraction of "a directory is a
key/value store, filenames are the keys, file contents are the values".

We could forbid `/` in tags and replace them with `.` or `_` or something,
but that's a bit sad because i like `/` as the "hierarchy separator".

Wikipedia claims, and experiment confirms:
> The big solidus ⧸ (Unicode code point U+29F8) is permitted in Unix
> and Windows filenames.

<https://en.wikipedia.org/wiki/Filename#Problematic_characters>

But that feels like madness.

I think the least worst option is to replace `/` with `.` in tags.
Opinions?