feat: let SDK dependency drive coglet installation instead of Dockerfile generator
Coglet is now only installed explicitly in the Dockerfile when there's a
specific source (COGLET_WHEEL env var, local file, or pinned PyPI version).
Otherwise, pip install cog handles it — cog >= 0.17.0 declares coglet as
a hard dependency, older versions don't pull it in.
Also prevents coglet from being installed alongside legacy SDK < 0.17.0,
even when a coglet wheel is auto-detected from dist/.