Repo cleanup: drop stale design docs and sim_results corpus

Remove 24 root-level design/notes markdown files (kept README.md and
gogobee_revisit_plan.md) plus the entire sim_results/ tree (~68M of H/J
phase baselines, traces, summaries, and helper scripts). Ignore
sim_results/ going forward — re-runs can regenerate it.

.gitignore

@@ -8,3 +8,4 @@ gensolver
 holdem-train
 /open5e-import
 /expedition-sim
+sim_results/