Give experiments IDs (P0, E0-E11): registry, per-experiment layout, spec moved to docs/experiments/P0-pilot.md

Raw pilot data stays ignored (copyrighted text); blind key and numeric results are tracked.
Runner notes move into docs/infrastructure.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FYqwNMyPUHeTbFWg83GQq6

tests/test_pilot_scripts.py

@@ -2,7 +2,7 @@
 import importlib.util
 from pathlib import Path
 
-ROOT = Path(__file__).resolve().parents[1] / "experiments" / "pilot"
+ROOT = Path(__file__).resolve().parents[1] / "experiments" / "P0-pilot"
 
 
 def load(name):