Add MatrixTransport (A1) — step-2 PR B

- matrix.py: Transport implementation over the Matrix client-server
  HTTP API directly (token auth, /sync long-poll, /send) using the
  existing httpx dependency — no Matrix SDK; four endpoints total.
  Deviation from the presented plan (matrix-nio) flagged in the PR
- First sync learns position and skips backlog, so room history is
  never delivered as context (A1); own messages and non-message
  events filtered; sender MXID passed through for the orchestrator's
  operator allowlist
- Sync loop backs off (capped at 60s) and retries indefinitely on
  homeserver errors, mirroring B2's polling posture
- Encrypted rooms unsupported: bot rooms are created unencrypted
  until E2E lands (operator decision; room recreated accordingly)
- NISSE_MATRIX_TEST_ROOM_ID added: the live smoke test binds only
  to the dedicated test room, never the production room
- 8 unit tests over a mocked HTTP layer; live integration test
  verified against the real homeserver

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/integration/test_matrix_live.py

@@ -0,0 +1,49 @@
+"""Live smoke test against the real homeserver, bound strictly to the
+dedicated test room (never the production room). Skips unless the Matrix env
+vars, including NISSE_MATRIX_TEST_ROOM_ID, are present."""
+
+import os
+import uuid
+from pathlib import Path
+
+import pytest
+
+from nisse_dev.envfile import load_env_file
+from nisse_dev.matrix import MatrixConfig, MatrixTransport
+
+load_env_file(Path(__file__).resolve().parents[2] / ".env")
+
+REQUIRED = (
+    "NISSE_MATRIX_HOMESERVER",
+    "NISSE_MATRIX_USER_ID",
+    "NISSE_MATRIX_TOKEN",
+    "NISSE_MATRIX_TEST_ROOM_ID",
+)
+
+pytestmark = [
+    pytest.mark.integration,
+    pytest.mark.skipif(
+        not all(os.environ.get(var) for var in REQUIRED),
+        reason=f"live matrix config absent; set {', '.join(REQUIRED)}",
+    ),
+]
+
+
+def test_send_lands_in_test_room() -> None:
+    config = MatrixConfig.from_env(room_var="NISSE_MATRIX_TEST_ROOM_ID")
+    marker = f"transport smoke test {uuid.uuid4().hex}"
+    transport = MatrixTransport(config)
+    try:
+        transport.send_message(marker)
+        response = transport._http.get(
+            f"/rooms/{config.room_id}/messages",
+            params={"dir": "b", "limit": "10"},
+        )
+        response.raise_for_status()
+        bodies = [
+            event.get("content", {}).get("body")
+            for event in response.json().get("chunk", [])
+        ]
+        assert marker in bodies
+    finally:
+        transport.close()