chore: gitignore core dumps (core.<pid>)

Stop crashed-process core dumps (core.144926, etc.) from showing up as
untracked noise / being committed by accident.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitignore

@@ -237,6 +237,10 @@ package.json
 package-lock.json
 .worktrees/
 .gstack/
+
+# Core dumps from crashed processes (e.g. core.12345)
+core.[0-9]*
+**/core.[0-9]*
 .gitnexus
 
 # graphify knowledge graph outputs