chore(gitignore): never track wallet.dat (holds private keys)

A wallet.dat placed in the repo root for recovery was untracked but NOT ignored,
so a stray 'git add .' could commit private keys. *.bak already covered the
salvage backups; add wallet.dat / wallet-*.dat / wallet.dat.* explicitly.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -33,6 +33,10 @@ imgui.ini
 *.bak*
 *.params
 asmap.dat
+# Wallet files hold PRIVATE KEYS — never commit them
+wallet.dat
+wallet-*.dat
+wallet.dat.*
 /external/drg-xmrig
 /memory
 /todo.md