Add COG_WHEEL env var parsing and dual wheel embedding
- Create pkg/dockerfile/wheel.go with WheelSource enum and WheelConfig
- Add ParseCogWheel() to parse COG_WHEEL env var values:
cog, coglet, coglet-alpha, URLs, or file paths
- Add GetWheelConfig() to determine wheel based on COG_WHEEL and cog_runtime flag
- Create pkg/wheels/ package with simple go:embed for cog.whl and coglet.whl
- Add script/generate-wheels to build both wheels to dist/ and copy to pkg/wheels/
- Add script/build-coglet-server for linux/amd64 coglet Go binary
- Simplify Makefile: remove complex wheel targets, use go generate
- Update StandardGenerator and migrator to use wheels.ReadCogWheel()
Closes cog-mrj, cog-a86, cog-i3w
