Problem / Motivation

install.sh had several UX issues and silent bugs that caused configuration to be incomplete or confusing on first run:

• MLflow experiment creation silently failed: the script called databricks experiments create-experiment, which doesn't exist in the Databricks
  CLI. The error was swallowed by 2>/dev/null, leaving MLFLOW_EXPERIMENT_ID empty and MLflow showing as disabled in the summary even when the user
  opted in.
• No fallback when an experiment already exists: re-running the installer with the same experiment path would fail the create call with no
  recovery.
• LLM "Other" path had a misleading default: when manually entering a custom endpoint, the input field defaulted to databricks-claude-sonnet-4-6
  — the model the user explicitly chose not to use.
• APP_NAME_DEFAULT used before app name was collected: the auto-created MLflow experiment path was built from a hardcoded genie-workbench string,
  not the user's chosen app name (which is collected two steps later).
• Inconsistent Y/N prompt formatting: some prompts showed [Y/n], others [y/N], with no visual consistency.
• No Enter-to-accept for the recommended model: Step 5 required typing a number even to pick the clearly recommended option.

Proposed Solution

• Replace databricks experiments create-experiment with POST /api/2.0/mlflow/experiments/create (the REST API pattern already used elsewhere in
  the script), with a search-by-name fallback so re-runs succeed when the experiment already exists.
• Remove the databricks-claude-sonnet-4-6 default from the manual endpoint entry prompt.
• Remove APP_NAME_DEFAULT and replace the hardcoded experiment path with a prompted input so users can match the path to their app name.
• Standardize all Y/N hints to [Y/N].
• Add default index support to _select_from so pressing Enter selects the recommended option in Step 5.
• Change the MLflow enable prompt default to Y (recommended on).

Alternatives Considered

• Keeping databricks experiments create-experiment and handling CLI version differences — rejected in favor of the REST API which is already the
  pattern in this script and more stable across CLI versions.

Additional Context

All changes are in scripts/install.sh. No deploy logic or backend code was modified.