Add a Prompt Registry discoverability step to mlflow-onboarding

[adamgurary]

Summary

No step in mlflow-onboarding covered how to register a Unity Catalog-backed prompt with mlflow.genai.register_prompt or how to make registered prompts discoverable in the MLflow UI.

A user following the existing onboarding skill would register a prompt successfully but find:

• No UI link returned or logged
• The experiment's Prompts tab empty
• The schema picker showing "nothing to select"

The root cause is that the Prompts tab requires the mlflow.promptRegistryLocation experiment tag to be set, and nothing in the skill mentioned this. Recent mlflow versions set this tag automatically during register_prompt, but the skill gave no guidance on this behavior or the manual fallback for older versions.

Changes

Added step 4 ("Prompt Registry") to the GenAI Integration section of mlflow-onboarding/SKILL.md:

• Notes that the workspace Prompt Registry preview must be enabled (account admin → Previews) for the UI to show prompts
• Shows mlflow.genai.register_prompt(name="catalog.schema.my_prompt", template=...) for a UC-backed prompt
• Notes that register_prompt logs a UI link on success and auto-links the active experiment's Prompts tab to the schema (via the mlflow.promptRegistryLocation tag) on recent mlflow versions
• Shows prompt_version.uri (prompts:/catalog.schema.my_prompt/1) as the programmatic handle to load the prompt later
• Includes a fallback for older mlflow: if the Prompts tab is empty, set mlflow.promptRegistryLocation manually with mlflow.set_experiment_tag