cli: suggest the closest flag or command on a typo

An unknown --flag now logs the nearest accepted spelling within two
edits as did_you_mean, painted green next to the red typo in the pretty
log format; logfmt stays plain. A bare first argument within two edits
of a command name is rejected with the same hint instead of being
fetched as a url.

The Levenshtein helper moves from SlashCommand into string.zig so the
agent REPL and the CLI share it, with the table widened to fit the
longest flag name.