Simplify SQLMesh to use single prod gateway with virtual environments

- Remove dev gateway (local DuckDB file no longer needed)
- Single prod gateway connects to R2 Iceberg catalog
- Use virtual environments for dev isolation (e.g., dev_<username>)
- Update CLAUDE.md with new workflow and environment strategy
- Create comprehensive transform/sqlmesh_materia/README.md

Benefits:
- Simpler configuration (one gateway instead of two)
- All environments use same R2 Iceberg catalog
- SQLMesh handles environment isolation automatically
- No need to maintain local 13GB materia_dev.db file
- before_all hooks only run for prod gateway (no conditional logic needed)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

transform/sqlmesh_materia/config.yaml

@@ -1,18 +1,8 @@
 # --- Gateway Connection ---
+# Single gateway connecting to R2 Iceberg catalog
+# Local dev uses virtual environments (e.g., dev_<username>)
+# Production uses the 'prod' environment
 gateways:
-
-  dev:
-    connection:
-      # For more information on configuring the connection to your execution engine, visit:
-      # https://sqlmesh.readthedocs.io/en/stable/reference/configuration/#connection
-      # https://sqlmesh.readthedocs.io/en/stable/integrations/engines/duckdb/#connection-options
-      type: duckdb
-      database: materia_dev.db
-      extensions:
-        - name: zipfs
-        - name: httpfs
-        - name: iceberg
-
   prod:
     connection:
       type: duckdb
@@ -21,8 +11,7 @@ gateways:
         - name: httpfs
         - name: iceberg
 
-
-default_gateway: dev
+default_gateway: prod
 
 # --- Hooks ---
 # Run initialization SQL before all plans/runs