Initial commit: rust-craft plugin

Rust-on-NixOS knowledge plugin. Relocates plugin.json into .claude-plugin/
so it loads as a proper plugin, and adds README/LICENSE/.gitignore for
distribution. Skill rust-nix-toolchain (Cranelift vs LLVM codegen, fenix
nightly pins, cargo/clippy gates) with cranelift-limitations reference.

CLAUDE.md

@@ -0,0 +1,7 @@
+# rust-craft Plugin
+
+Rust development knowledge for NixOS environments — toolchain management, codegen backends, and common pitfalls.
+
+## Skills
+
+- **rust-nix-toolchain**: Cranelift vs LLVM codegen backend issues, NixOS fenix toolchain management, cargo quality gate patterns