docs: remove stale Snowflake semantic view references

## Context

Cleanup #113 intentionally removed the unused Snowflake Cortex assets:

- `snowflake/semantic_view.yml`
- `snowflake/README.md`

However, `models/semantic/README.md` still refers to both files and describes
the Snowflake semantic view/Cortex Analyst workflow as if it were supported by
this repository.

The stale references were noticed while reviewing #99. This cleanup is outside
the scope of #99 and is tracked separately at @daria-sukhareva’s request.

## Problem

The README currently contains references that no longer match the repository:

| Reference | Path | Current state |
|---|---|---|
| Snowflake semantic view YAML | `snowflake/semantic_view.yml` | Removed in #113 |
| Snowflake semantic documentation | `snowflake/README.md` | Removed in #113 |

This is documentation drift, not a dbt build failure. Contributors and agents
may look for files and workflows that are no longer supported.

## Expected change

Update `models/semantic/README.md` so it describes only the semantic-layer
integrations currently supported by the repository.

## Acceptance criteria

- [ ] Remove the reference to `snowflake/semantic_view.yml` from the Contents section.
- [ ] Remove the Snowflake Cortex Analyst usage section and its reference to `snowflake/README.md`.
- [ ] Remove the Snowflake-specific screenshot if it represents the removed workflow.
- [ ] Confirm that no remaining documentation references the deleted Snowflake files.
- [ ] No dbt model, metric, or SQL changes.

## Out of scope

- Restoring Snowflake Cortex assets.
- Changing `models/semantic/semantic_models.yml`.
- Changes related to #99.
- Nao chat or ChatBI eval changes.

## Related

- Cleanup #113
- Discovered during #99 review