[chore:project] Add development planning files to .gitignore

Added `IMPROVEMENT_PLAN.md` and `DEVELOPER_GUIDE.md` to gitignore as
these are internal development planning documents that should not be
tracked in version control.

Author: Qu1nel

.gitignore

@@ -194,4 +194,5 @@ cython_debug/
 .cursorindexingignore
 
 # Development planning files
-IMPROVEMENT_PLAN.md
+IMPROVEMENT_PLAN.md
+DEVELOPER_GUIDE.md