Staging Codegen

The previous source-specific SQLMesh layout used generated *_staging.stg_* models. The active CDM workflow now writes SQLMesh models directly under transforms/main/models/environmental_observations/; no staging contracts are currently active.

python scripts/generate_staging.py --check remains in CI as a harmless drift gate for future forks that choose to add codegen-driven staging contracts. With no *_staging Soda contracts present, it has nothing to regenerate.

If a future source needs a reusable staging boundary, add a Soda contract with a source_table key and run python scripts/generate_staging.py. Prefer direct raw references from CDM models unless the normalization is repeated, complex, or serves as an intentional quality boundary.