.gitignore: skip cmd/ binaries (gensolver, holdem-train)

Both are 16MB Go binaries built from cmd/gensolver and cmd/holdem-train.
They were showing up as untracked on every status check. Also drops a
stray 0-byte internal/db/gogobee.db that someone created at the wrong
path (real DB lives in data/, already gitignored).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -3,3 +3,5 @@ data/
 .env.*
 !.env.example
 gogobee
+gensolver
+holdem-train