Add dispatch config unset to reset a setting to its default

[jongio]

dispatch config can list, get, set, edit, and print the path, but there is no way to clear one setting back to its default short of editing the file by hand or resetting the whole config. Add a config unset <key> subcommand that resets a single setting to its default value and saves.

Acceptance criteria

• dispatch config unset <key> resets that one key to its default and persists the change through the normal save path.
• After unset, config get <key> reports the default value.
• An unknown key returns the same error config get/set give, listing valid keys.
• config unset with no key, or more than one key, returns a clear usage error.
• Shell completion lists unset as a config subcommand and the README documents it.
• Tests cover a scalar reset, an unknown key, and the argument-count errors.

Complexity

S

Priority

Medium