Show usage on positional-arg errors

entire project create and other vendored entiredb plugin subcommands with required positional args printed only the bare cobra error ("accepts 1 arg(s), received 0") on failure, leaving the user to look up the right invocation themselves.

Extend main.go's existing error-routing switch (which already shows usage for unknown-command / unknown-flag) with an arg-count branch. Switch from ExecuteContext to ExecuteContextC so the deepest matched cobra.Command is available — that's the one whose Args validator returned the error, and it owns the right UsageString for the failure. Pass it to the existing showSuggestion path rather than the root command, so the printed usage matches the failing subcommand rather than the cli root.

The matcher is a substring check on "arg(s)" — cobra's stock validators (ExactArgs, NoArgs, MinimumNArgs, MaximumNArgs, RangeArgs) all surface that substring, and it doesn't appear in cobra's other error paths or in the cli's own errors, so it's a cheap discriminator without reaching into cobra internals.

Tested: entire project create → project create usage + error entire repo clone → repo clone usage + error entire foobarbaz → root usage + error (unchanged) entire project nonexistent → project usage + error (unchanged)