Rework prompt frontmatter (#12262)

Moved some things around so prompts now always have front-matter to
return, either by creating a prompt with default front-matter, or
bailing earlier on importing the prompt to the library.

In the future we'll improve visibility of malformed prompts in the
`prompts` folder in the prompt manager UI.

Fixes:

- Prompts inserted with the `/prompt` command now only include their
body, not the entire file including metadata.
- Prompts with an invalid title will now show "Untitled prompt" instead
of an empty line.

Release Notes:

- N/A