refactor(arg_tokenizer): add Token::is_free_positional(), dedupe 4 call sites

"Positional kind, and not consumed as some preceding flag's value" was
hand-spelled identically in git.rs (x2), golangci_cmd.rs, search.rs,
and dotnet_cmd.rs. Added Token::is_free_positional() to the shared type
and switched all 5 call sites to it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>