Start a plugin marketplace for Apple platform work

Skills have been living inside one app's repository, which means the next
Xcode project starts without them and a copy is the only way to share.
A marketplace makes them installable once at user scope.

Two kinds of knowledge, handled differently. What Apple ships with Xcode
stays Apple's: this repository carries a script that exports it from the
locally installed Xcode rather than a copy, so nobody redistributes
pre-release material and everyone's guidance matches their own SDK.

What we learned by shipping goes in the skill, and only if it burned us —
the file writes hidden in view bodies, the blobs that froze the main
thread, the watch target that quietly stopped compiling, the test summary
that reads as zero tests when eighty-four passed. Each of those cost real
time twice before being written down.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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