feat(cli): support flattened builds

[Parker J. Rule (pjrule)]

Summary

• add an opt-in --single-user-layer flag to langgraph build
• generate one bind-mounted RUN for source copy, dependency installation, API restoration, and build-tool cleanup
• preserve runtime ENV and WORKDIR in the resulting image config
• retain the existing layered generator for unsupported or more complex build shapes

Expected impact

In the Kubernetes BuildKit benchmark, combining the generated Python user steps into one non-empty filesystem layer reduced median C3 + gVisor solve time by about 10 seconds (54.93s to 44.76s). Fast samples showed roughly 20–24 seconds of structural savings. Compressed image size was effectively unchanged; the gain came from avoiding repeated native-snapshot copies.

This deliberately trades fine-grained warm-cache reuse for faster cold, short-lived builders.

Validation

• make format
• make lint
• make test (337 passed)
• Docker Buildx outline validation of a generated single-layer Dockerfile