docs: add Snowflake-only launch scope note to roadmap

## Task
Make the public roadmap explicit that Snowflake is the launch target and other warehouses are roadmap/design-partner work.

## Likely files
- `README.md`
- `docs/chukei_prd.md`
- chukei-docs roadmap page

## Acceptance criteria
- Snowflake is described as the validated target.
- Databricks is only described as roadmap, not launch support.