refactor: reorganize, add package (opencode, osgrep, beads), add ai agent shell

shells/rust.nix

@@ -0,0 +1,12 @@
+{ pkgs, ... }:
+
+pkgs.mkShell {
+  name = "rust";
+  packages = with pkgs; [ cargo rustc rust-analyzer rustfmt clippy ];
+  shellHook = ''
+    mkdir -p "$PWD/.cache/cargo"
+    export CARGO_HOME="$PWD/.cache/cargo"
+    echo "Rust toolchain shell ready"
+    echo "Rust: ${pkgs.rustc.version}"
+  '';
+}