fix: remove dead GameContext system and orphaned MemorySlotPicker

GameContext (Provider, hooks, context-create, types) was never wired into
the app — no layout or page wrapped children with GameProvider. The only
consumer, MemorySlotPicker, was itself orphaned (never imported/rendered).
The app uses direct Zustand hooks throughout. Removes 6 dead files.

Fixes #65

docs/dependency-graph.json

@@ -1,6 +1,6 @@
 {
   "_meta": {
-    "generated": "2026-05-18T13:14:03.496Z",
+    "generated": "2026-05-18T15:51:09.351Z",
     "description": "Import dependency graph for src/lib/game. Keys are files, values are arrays of files they import.",
     "usage": "To find what a file affects, search for its path in the VALUES. To find what a file depends on, look at its KEY entry."
   },