fix(scripts): add explicit positional binding to PowerShell create-new-feature params (#1885)

The $Number (Int32) parameter was implicitly receiving positional
arguments intended for $FeatureDescription, causing a
ParameterBindingArgumentTransformationException when AI agents
called the script with positional strings.

Add [Parameter(Position = 0)] to $FeatureDescription so it binds
first, and mark $Number with [Parameter()] (no Position) so it
only binds by name (-Number N).

Fixes #1879

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>