chore: standardize Nim builds to out/ subdir, simplify .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PPO_Bot_garage/PPO_Bot.sh

@@ -4,4 +4,4 @@
 # which deadlock when called from within a multi-threaded Nim bot process.
 export OPENBLAS_NUM_THREADS=1
 cd -- "$(dirname -- "$0")"
-exec "./PPO_Bot"
+exec "./out/PPO_Bot"

PPO_Bot_garage/config.nims

@@ -2,6 +2,7 @@
 # ponytail: adjust path per machine, or use pkg-config
 switch("passL", "-L/nix/store/v07svn2y92bvzjl51aj7c9ca1cwg7rw7-openblas-0.3.32/lib -lopenblas")
 switch("threads", "on")
+switch("outdir", "out")
 # begin Nimble config (version 2)
 when withDir(thisDir(), system.fileExists("nimble.paths")):
   include "nimble.paths"