`-Number` defaults to 0, so the previous `-eq 0` / `-ne 0` checks could
not distinguish an unset flag from an explicit `-Number 0`: a user
requesting branch `000-...` was silently routed into auto-detection.
Switch both checks to `$PSBoundParameters.ContainsKey('Number')`, which
tests whether the flag was actually supplied — mirroring the bash twin's
empty-string sentinel (`[ -z "$BRANCH_NUMBER" ]` / `[ -n ... ]`).
Add a parity regression test to both TestCreateFeatureBash and
TestCreateFeaturePowerShell asserting `--number 0` / `-Number 0` yields
`000-zero`.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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