plan(SP22): Task 1 review fixes — conftest fixture bug + Python 3.13 note

- conftest.py example now uses SAMPLE_837P constant so sample_837p_bytes
  reads the file (not the directory).
- Note that Python 3.13 satisfies requires-python >= 3.11 if 3.11 isn't
  installed.

docs/superpowers/plans/2026-06-21-cyclone-pipeline-agent.md

@@ -197,17 +197,19 @@ import pytest
 from pathlib import Path
 
 FIXTURES_DIR = Path(__file__).parent / "fixtures"
+SAMPLE_837P = FIXTURES_DIR / "sample_837p.edi"
 
 
 @pytest.fixture
 def sample_837p_path() -> Path:
     """Path to a small valid 837P file used across tests."""
-    return FIXTURES_DIR / "sample_837p.edi"
+    return SAMPLE_837P
 
 
 @pytest.fixture
 def sample_837p_bytes() -> bytes:
-    return FIXTURES_DIR.read_bytes()
+    """Contents of the sample 837P fixture, as bytes."""
+    return SAMPLE_837P.read_bytes()
 ```
 
 - [ ] **Step 4: Copy the minimal 837P fixture from cyclone**
@@ -253,6 +255,8 @@ python -c "import cyclone_pipeline; print('OK:', cyclone_pipeline.__version__)"
 
 Expected: `OK: 0.1.0`.
 
+Note: Python 3.13 also satisfies `requires-python = ">=3.11"` and is acceptable if 3.11 is not installed on the host.
+
 - [ ] **Step 8: Verify pytest can discover the test tree**
 
 ```bash