refactor: slim flake.nix into nix/ entry files

nix/devshell.nix

@@ -0,0 +1,18 @@
+# Dev shell: the rust toolchain plus the exact native build inputs
+# the crane derivations use (git, sqlite, pkg-config, cmake — see
+# ../rust.nix), so a plain `cargo build` / `cargo test` in the shell
+# sees the same toolchain shape as CI.
+{ pkgs, rust }:
+{
+  default = pkgs.mkShell {
+    packages =
+      rust.nativeBuildInputs
+      ++ (with pkgs; [
+        cargo
+        clippy
+        rust-analyzer
+        rustc
+        rustfmt
+      ]);
+  };
+}