docs: establish Go backend repository rules

data/incoming/README.md

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+# Incoming dataset
+
+Copy the challenge dataset to this directory as `challenge_data.csv.gz` before running the future importer. The archive is intentionally ignored by Git.
+
+Expected source columns:
+
+```text
+session_key, try_seq, terminal_key, merchant_key, category_id,
+category_title, amount, adjusted_fee, session_status, try_status,
+switch_response_code, psp_code, issuer_bank_code, payer_card_key,
+verify_type, init_time_ms, verify_time_ms, created_at, try_created_at,
+verified_at, settled_at, expire_in
+```
+
+Do not add merchant passwords or other secrets to this directory. Demo credentials will be generated/seeding inputs managed separately through secure environment or one-off administrative workflows.
+