Skip redirect sources that already exist when cutting a release

The nav patch step in cut-release adds a dated sibling redirect for
every /specification/draft/... entry, but it never checked whether that
dated source was already present. When main already carries the
<version>/ redirects, the loop appends a second copy of each source and
Mintlify rejects the resulting docs.json for reusing a source path,
which is exactly what broke the 2026-07-28 release PR. Track the
existing sources in a set and skip any collision so the step is
idempotent regardless of what main already contains.

:house: Remote-Dev: homespace