fix(extensions): prevent omp-hooks from crashing pi at startup (#119)

Remove the load-time pi.setLabel() call and port the bigpowers_skill tool
schema from the non-existent pi.zod to TypeBox — two fork-API artifacts
that aborted pi startup for every package consumer. Declare pi.extensions
(the key pi reads), drop the dead omp.extensions key, harden the OMP smoke
test to model pi's load-phase guard and run it in CI. Verified against real
pi 0.85.1: 81 commands + bigpowers_skill tool load with 0 errors.