fix: untrack runtime config files and add them to .gitignore

These files are machine-specific and generated at runtime — they should
never have been committed. Remove from index (files kept on disk) and
add explicit gitignore rules to prevent future re-adds.

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

config/dns/Corefile

@@ -1,12 +0,0 @@
-. {
-    forward . 8.8.8.8 1.1.1.1
-    cache
-    log
-    health
-}
-
-dec {
-    file /data/dec.zone
-    log
-}
-