Add Phase 0a black-box integration test suite

81 tests across 11 files plus a 5-module harness (load, pg, streams,
containers, payloads), covering all eleven Phase 0 goals and all 13 named
chaos-flag pathologies. Talks to the system only over HTTP, Postgres, Redis,
and the Docker CLI -- no imports of application code. RED-BASELINE.md records
the suite failing red against the empty repo before any implementation
existed; README.md documents the request-contract fields invented to fill
gaps in the spec, measurement methodology, and known-flaky tests.

This suite is a frozen contract from this point forward per CLAUDE.md.

tests/harness/payloads.py

@@ -0,0 +1,28 @@
+"""The POST /api/transactions request shape this suite assumes.
+
+Neither spec pins down the request field names for creating a transaction --
+only the response envelope (phase-0-base-project-spec.md §6.1) is specified.
+Since the test suite is the contract, this module fixes the shape Agent B's
+implementation must accept. See tests/README.md, "Request contract
+assumptions", for the rationale.
+"""
+
+
+def tx_payload(
+    account_id,
+    kind="deposit",
+    amount_minor=1000,
+    currency="EUR",
+    merchant_name="Test Merchant",
+    category_id=None,
+):
+    payload = {
+        "account_id": account_id,
+        "kind": kind,
+        "amount_minor": amount_minor,
+        "currency": currency,
+        "merchant_name": merchant_name,
+    }
+    if category_id is not None:
+        payload["category_id"] = category_id
+    return payload