chore(governance): land missing docs/packaging before the standards#505 gates bite (#5)

Remediates this repo against the two governance gates promoted from
advisory to blocking in
[hyperpolymath/standards#513](hyperpolymath/standards#513)
(closing standards#505).

Those gates previously **could not fail**: the docs check only emitted
`::warning::`, and the package-policy job ended in an unconditional `✅
Package policy check passed`. They are now real, with a grace window —
they begin **blocking on 2026-08-21**. This repo was measured as
non-compliant, so this PR lands the missing files ahead of that date.

**Added here:** `packaging`

### `flake.nix`
A **dev shell**, not a package build. Estate policy is Guix primary /
Nix fallback; this is the fallback tier, and a `guix.scm` is welcome
later to move to the primary.

- Packages mirror the tooling **actually present in this repo**
(`just`), not a generic default.
- `nixpkgs` is pinned to an exact revision per the estate SHA-pinning
rule.
- **Verified**: `nix flake check` passes (nix 2.35.1). Every package
name was confirmed to exist in the pinned revision before generation —
nothing here is guessed.

---
Part of an estate-wide sweep; 63 repos were identified as non-compliant.
Third-party/forked repos (`HOL`, `awesome-*`, `lua-filters`) were
**excluded** per the licence policy's do-not-touch rule.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: hyperpolymath

flake.nix

@@ -0,0 +1,34 @@
+# SPDX-License-Identifier: MPL-2.0
+# SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell
+#
+# Development environment for gnpl.
+#
+# Estate policy is Guix primary / Nix fallback (hyperpolymath/standards).
+# This is the Nix fallback tier. It is a dev shell, not a package build:
+# it declares the toolchain needed to work on this repo, pinned to an
+# exact nixpkgs revision per the estate SHA-pinning rule.
+#
+# Packages mirror the build tooling actually present in this repo
+# (just) — not a generic estate default.
+#
+#   nix develop      # enter the shell
+#   nix flake check  # verify this file evaluates (run before committing)
+{
+  description = "gnpl development environment";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/b134951a4c9f3c995fd7be05f3243f8ecd65d798";
+
+  outputs = { self, nixpkgs }:
+    let
+      systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+      forAllSystems = f:
+        nixpkgs.lib.genAttrs systems (system: f nixpkgs.legacyPackages.${system});
+    in
+    {
+      devShells = forAllSystems (pkgs: {
+        default = pkgs.mkShell {
+          packages = with pkgs; [ just ];
+        };
+      });
+    };
+}