ci: check `metadata.toml` files

[ee7]

The prob-specs ci.yml workflow checks Markdown and JSON files, but not the metadata.toml files.

In the hopefully-unlikely event that a strictly invalid metadata.toml file reaches main, configlet sync would produce an error to complain about the invalid TOML when the syncing scope includes metadata.

We could check that:

• every exercise has a metadata.toml file
• with a valid title key/value pair
• with a valid blurb key/value pair
• if the source key is present, the value is valid
• if the source_url key is present, the value is valid
• there is no other key/value pair (it may be a typo of source or source_url, like source-url)