[.gitignore] Ignore .claude and .gemini in subdirectories (llvm#167029)

Currently `.claude/` and `.gemini/` are only ignored in the root of the
repo. Developers might conceivable run these tools in project
subdirectories, in which case these should be ignored as well.

Author: JDevlieghere

.gitignore

@@ -54,9 +54,9 @@ autoconf/autom4te.cache
 /cmake-build*
 # Coding assistants' stuff
 /CLAUDE.md
-/.claude/
+.claude/
 /GEMINI.md
-/.gemini/
+.gemini/
 
 #==============================================================================#
 # Directories to ignore (do not add trailing '/'s, they skip symlinks).