Add reproducible Nix dev environment

Replace the Python venv with a Nix devshell pinning ansible-core 2.21.3,
ansible-lint, yamllint and a Python with proxmoxer/requests. The Python
dependencies share the interpreter that runs ansible, so pve-*.yml plays
on implicit localhost can import proxmoxer without inventory changes.

The shellHook exports absolute ANSIBLE_CONFIG, ANSIBLE_INVENTORY,
ANSIBLE_ROLES_PATH and ANSIBLE_COLLECTIONS_PATH, so commands work from
the repository root as well as from ansible/.

Also un-ignore .envrc, which the global gitignore hides, and ignore the
stray .ansible/ runtime directory.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GTocXkGUUazHdKKd3r9k71

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1787498568,
+        "narHash": "sha256-9i/VTdusq/+NM/tz+J1Re+ojkMB8MBf0QshnYfzHz30=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "56c02bc00adcf003215cc4bd996d6efaf4cff188",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}