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What is the idea?

conda create --file explicit.txt supports installing from an explicit specs file, but this format is handled directly by the CLI and not through the environment spec plugin system introduced for extensibility.

Why is this needed?

This brings feature parity between conda env and conda create/install/update by allowing all environment specification formats—including explicit files—to be handled uniformly via plugins.

What should happen?

Allow explicit spec files (e.g. --file explicit.txt) to be handled by the environment spec plugin interface. This would enable support for explicit files to be provided as a plugin, similar to other environment spec formats.

Additional Context

Refs #14684 and conda/conda-planning#15