fix: Update .gitignore, add .remember and .cmux

Tidy up .gitignore by removing a placeholder header and adjusting spacing/comments. Add entries to ignore .remember and .cmux directories, and retain other existing ignore patterns.

Author: kimchanhyung98

.gitignore

@@ -1,5 +1,3 @@
-# DEFAULT
-
 # @todo
 
 # Distribution / packaging
@@ -156,6 +154,7 @@ $RECYCLE.BIN/
 !.claude/memory/.gitkeep
 .claude/plans
 .claude/settings.local.json
+.remember
 
 # Codex
 .codex/commands/speckit.*
@@ -175,3 +174,6 @@ agent-os/
 .specify/
 .speckit/
 specs/
+
+# etc
+.cmux