Replace mockery mocks with AI-written fakes

Changes

MockImageStore and MockVersionLookup were the only mocks in active use. Replace them with fakeImageStore and fakeVersionLookup in a new fakes_test.go. Remove all mocks.go files, .mockery.yaml, and the mockery toolchain from the Makefile.

Why

Claude-code struggles to refresh mocks when changing branches. Instead of calling make clean and make mocks every branch change, it's easier if the required test mocks/fake comes with the code on every branch switch. AI can easily create fakes now, suggest we do this instead of using mockery.