Support sdk_version: "prerelease" sentinel in cog.yaml (#2862)
Adds a special "prerelease" value for build.sdk_version that generates
`uv pip install --pre cog` without a version pin. This lets testers
always get the latest pre-release SDK without updating cog.yaml each
time a new RC is cut.
When stable ships, builds automatically move to stable. A build-time
warning nudges users to pin a specific version for production.
