fix: remove target-cpu=native from tracked cargo config

.cargo/config.toml set target-cpu=native for x86_64-unknown-linux-gnu,
causing SIGILL in CI when build scripts compiled with AVX instructions
ran on runners without AVX support. rust-cache amplified this by
sharing binaries across runners with different CPU feature sets.

Move .cargo/ to .gitignore so developers can opt into target-cpu=native
locally without affecting CI builds.

Author: YuminosukeSato

.cargo/config.toml

@@ -3,6 +3,3 @@ rustflags = ["-C", "target-cpu=native"]
 
 [target.aarch64-apple-darwin]
 rustflags = ["-C", "target-cpu=native"]
-
-[target.x86_64-unknown-linux-gnu]
-rustflags = ["-C", "target-cpu=native"]

.gitignore

@@ -13,3 +13,4 @@ z-ai/
 .claude/
 *.DS_Store
 site/
+.cargo/