Simplify wheel build and embed workflow
- Add script/build-wheels to build both cog and coglet wheels to dist/
- Replace go:generate script call with simple cp commands
- Delete script/generate-wheels (no longer needed)
- Add 'make wheel' target
- Simplify CI: use merge-multiple to download all artifacts at once
- Remove COG_WHEEL/COGLET_WHEEL env var setup from CI jobs
