Build slime and Miles base images from Modal named Images by default

[joyliu-q]

Successor to #381: instead of an opt-in named_image recipe arg, the launchers now resolve their base image via Image.from_name(...) unconditionally.

# slime launcher
Image.from_registry("slimerl/slime@sha256:a97e...")   # before
Image.from_name("slimerl/slime:nightly-dev-20260722a")  # after (SLIME_IMAGE)

# miles launcher
Image.from_registry(miles.docker_image)  # before
Image.from_name(miles.docker_image)      # after ("radixark/miles:dev-202608051303")

• The built-in patches / entrypoint reset / HF-cache cleanup still layer on top via run_commands, so behavior is otherwise unchanged — only the base resolution changes from a registry pull to a named-Image lookup.
• The old digest pin is kept as SLIME_REGISTRY_IMAGE and used by scripts/publish_framework_images.py, which builds each registry image once and publishes it under the named ref (slimerl/slime:nightly-dev-20260722a, radixark/miles:dev-202608051303). Names with / publish under an environment prefix (slimerl, radixark), which must exist.
• scripts/fetch_slime_patch_snapshots.py keeps reading from the registry digest.

Deployment note: the names must be published once (uv run scripts/publish_framework_images.py) before launches resolve them — from_name fails until then. Bumping to a newer upstream tag (e.g. slimerl/slime:nightly-dev-20260804a) is a constant change plus a republish.

Checklist

N/A — library change, not an example. Ran ruff check, ruff format, compileall, pre-commit, and the recipe/CLI test subset (230 passed).

Link to Devin session: https://modal.devinenterprise.com/sessions/ecdc4eabe1a0494bba278a0a786d7d87
Requested by: @joyliu-q

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Documentation preview: https://modal-labs-training-gym--training-gym-previews-preview-r-78da35.modal.run/386/docs