feat: altimate review --dialect flag (avoid silent snowflake default with no manifest)

From the v0.8.0 End-User release review. With no manifest, reviewPullRequest defaults the SQL dialect to snowflake (run.ts), which can mis-analyze a BigQuery/Redshift project on a manifest-less local run. Add a --dialect CLI option mirroring the .altimate/review.yml field, and surface the auto-default in the degraded banner. Docs caveat already added in v0.8.0.