Add Nix flake for dev environment

Provides PHP with imagick, pdo_mysql, pdo_sqlite extensions, composer,
and sqlite CLI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1770843696,
+        "narHash": "sha256-LovWTGDwXhkfCOmbgLVA10bvsi/P8eDDpRudgk68HA8=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "2343bbb58f99267223bc2aac4fc9ea301a155a16",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixpkgs-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}