Build function images with Nix instead of the up CLI #10

What problem are you facing?

The up CLI's Python function builder tars each function directory (following symlinks) and injects it into a base image. This works for simple functions but creates friction for a project of Modelplane's size:

• Shared library requires symlinks. The top-level lib/ directory is symlinked into each function directory so functions can import shared code. This is a workaround, not a feature.
• No third-party packages. Functions are limited to the standard library and the function SDK. We store CRDs as JSON instead of YAML because we can't depend on pyyaml.
• Type checking in CI doesn't work. Function directories aren't Python packages on disk — they have no __init__.py or pyproject.toml, so type checkers like pyright and ty can't resolve relative imports. IDE-based type checking generally works, but we can't run type checks in CI.

How could Modelplane help solve your problem?

Once crossplane/cli#21 lands, switch to building function runtime images with Nix (or Docker) and passing them to the up CLI as pre-built images. The CLI would skip the build step and just add the Crossplane package metadata layer.

This lets us structure the project as a standard Python monorepo — a real package with pyproject.toml, dependencies declared in one place, shared library installed via normal packaging, type checking and test runners that work out of the box.

Blocked on crossplane/cli#21.