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/fct_cot_positioning.sql

@@ -1,8 +1,8 @@
 -- Foundation fact: CFTC COT positioning, weekly grain, all commodities.
 --
--- Casts raw varchar columns to proper types, cleans column names,
--- computes net positions (long - short) per trader category, and
--- deduplicates via hash key. Covers all commodities — filtering to
+-- Reads directly from the landing zone, casts varchar columns to proper types,
+-- cleans column names, computes net positions (long - short) per trader category,
+-- and deduplicates via hash key. Covers all commodities — filtering to
 -- a specific commodity happens in the serving layer.
 --
 -- Grain: one row per (cftc_commodity_code, report_date, cftc_contract_market_code)
@@ -19,7 +19,19 @@ MODEL (
   cron '@daily'
 );
 
-WITH cast_and_clean AS (
+WITH src AS (
+  SELECT * FROM read_csv(
+    @cot_glob(),
+    compression   = 'gzip',
+    header        = true,
+    union_by_name = true,
+    filename      = true,
+    all_varchar   = true,
+    max_line_size = 10000000
+  )
+),
+
+cast_and_clean AS (
   SELECT
     -- Identifiers
     trim(market_and_exchange_names)                          AS market_and_exchange_name,
@@ -103,7 +115,7 @@ WITH cast_and_clean AS (
       prod_merc_positions_long_all,
       prod_merc_positions_short_all
     )                                                       AS hkey
-  FROM raw.cot_disaggregated
+  FROM src
   -- Reject rows with null commodity code or malformed date
   WHERE trim(cftc_commodity_code) IS NOT NULL
     AND len(trim(cftc_commodity_code)) > 0