refactor(transform): remove raw layer, read landing zone directly

- Delete 6 data raw models (coffee_prices, cot_disaggregated, ice_*,
  psd_data) — pure read_csv passthroughs with no added value
- Move 3 PSD seed models raw/ → seeds/, rename schema raw.* → seeds.*
- Update staging.psdalldata__commodity: read_csv(@psd_glob()) directly,
  join seeds.psd_* instead of raw.psd_*
- Update 5 foundation models: inline read_csv() with src CTE, removing
  raw.* dependency (fct_coffee_prices, fct_cot_positioning, fct_ice_*)
- Remove fixture-based SQLMesh test that depended on raw.cot_disaggregated
  (unit tests incompatible with inline read_csv; integration run covers this)
- Update readme.md: 3-layer architecture (staging/foundation → serving)

Landing files are immutable and content-addressed — the landing directory
is the audit trail. A raw SQL layer duplicated file bytes into DuckDB
with no added value.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

transform/sqlmesh_materia/models/foundation/dim_commodity.sql

@@ -6,8 +6,8 @@
 -- As new commodities are added (cocoa, sugar), rows are added here.
 --
 -- References:
---   usda_commodity_code → raw.psd_alldata.commodity_code (numeric string, e.g. '0711100')
---   cftc_commodity_code → raw.cot_disaggregated.cftc_commodity_code (3-char, e.g. '083')
+--   usda_commodity_code → staging.psdalldata__commodity.commodity_code (numeric string, e.g. '0711100')
+--   cftc_commodity_code → foundation.fct_cot_positioning.cftc_commodity_code (3-char, e.g. '083')
 --
 -- NOTE: Defined as FULL model (not SEED) to guarantee leading-zero preservation.
 --   Pandas CSV loading converts '083' → 83 even with varchar column declarations.