chore: gitignore /_opam/ local opam switch

A project-local opam switch (created for a working OCaml toolchain) is a
machine-specific environment dir and must not be tracked, mirroring the
existing /_build/ ignore.

https://claude.ai/code/session_01FkzAgzpZFSGxzorVNZ9FUF

Author: claude

.gitignore

@@ -13,6 +13,7 @@ Thumbs.db
 # Build
 /target/
 /_build/
+/_opam/
 /build/
 /dist/
 /out/