docs.to-html doesn’t support tilde expansion

[sellout]

Describe and demonstrate the bug

transcript:

.> project.create test-4402
test-4402/main> builtins.merge

{{A doc directly in the namespace.}}
some.ns.direct = 1

{{A doc pretty deeply nested in the namespace.}}
some.ns.pretty.deeply.nested = 2

{{A doc outside the namespace.}}
some.outside = 3

test-4402/main> add

Now, running this command will create a directory named ~ in whatever directory you ran ucm in, rather than expanding ~ to be the value of $HOME.

test-4402/main> docs.to-html some.ns ~/test-4402

Environment (please complete the following information):

• ucm --version: trunk on 2024-06-20
• OS/Architecture: macOS 14.5, M1

Additional context
This is extracted from #4402, since that report covered two separate issues.