fix(airflow): simplify kafka_load params and align docs

- Why:

  - For DE task we only need full ingest or limit-based sample.

  - load_* and full_load params were redundant and unclear in current flow.

- What:

  - Remove full_load and load_* params from kafka_load DAG contract.

  - Simplify kafka helpers (validate/check files) to fixed 4-stream ingest.

  - Sync AGENTS, README, runbook, architecture and airflow plan docs.

- Check:

  - python3 -m py_compile dags/kafka_load_dag.py dags/utils/kafka_helpers.py

  - Airflow smoke/full runs: ddl_init -> kafka_load -> etl_pipeline (all success).

  - Legacy path: make data && make transform (success).

README.md

@@ -106,7 +106,7 @@ flowchart TB
             DAG[DAG: ddl_init / kafka_load / etl_pipeline]
         end
 
-    Sources -->|make data| Kafka -->|MV| STG -->|Batch SQL| ODS -->|Batch SQL| DDS -->|VIEW| DM
+    Sources -->|kafka_load / make data| Kafka -->|MV| STG -->|Batch SQL| ODS -->|Batch SQL| DDS -->|VIEW| DM
     DAG -.->|оркестрация| STG & ODS & DDS & DM
 ```