Add Nix flake for building and installing on NixOS.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,61 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1774273680,
+        "narHash": "sha256-a++tZ1RQsDb1I0NHrFwdGuRlR5TORvCEUksM459wKUA=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "fdc7b8f7b30fdbedec91b71ed82f36e1637483ed",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,43 @@
+{
+  description = "sgard — Shimmering Clarity Gardener: dotfile management";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in
+      {
+        packages = {
+          sgard = pkgs.buildGoModule {
+            pname = "sgard";
+            version = "0.1.0";
+            src = pkgs.lib.cleanSource ./.;
+            subPackages = [ "cmd/sgard" ];
+
+            vendorHash = "sha256-uJMkp08SqZaZ6d64Li4Tx8I9OYjaErLexBrJaf6Vb60=";
+
+            ldflags = [ "-s" "-w" ];
+
+            meta = {
+              description = "Shimmering Clarity Gardener: dotfile management";
+              mainProgram = "sgard";
+            };
+          };
+
+          default = self.packages.${system}.sgard;
+        };
+
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            go
+            golangci-lint
+          ];
+        };
+      }
+    );
+}