docs: add ng_materialize_prompts to CLI reference

[JOBEBOLDER]

Fixes #1347

Summary

ng_materialize_prompts was missing from the CLI reference page. Users could only discover it through data/prompt-config.md.

Changes

• docs/reference/cli-commands.md: Added ng_materialize_prompts / nemo_gym_materialize_prompts with full parameter table and example, including a note explaining when to use it versus ng_prepare_data

Acceptance Criteria

• ng_materialize_prompts appears in the CLI reference with usage and parameters
• A user can understand the difference between the data preparation commands from the docs

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