refactor: slim flake.nix into nix/ entry files

nix/reference-docs.nix

@@ -1,41 +0,0 @@
-{
-  stdenv,
-}:
-
-# The repo `docs/` markdown tree, shipped as a standalone derivation so
-# agents can read the reference docs in-container (added as a claude
-# additional directory by the harness) WITHOUT pulling the branding +
-# prompt assets they don't need, and so the `hyperhive/website` repo can
-# reuse the exact same tree as a flake input.
-#
-# Pure data: the docs are copied verbatim (never transformed), so there
-# is no writable/build step — `$out` is the docs tree as-is.
-#
-# Output layout:
-#   $out/  — the repo `docs/` tree verbatim (e.g. `$out/setup.md`)
-
-stdenv.mkDerivation {
-  pname = "hyperhive-docs";
-  version = "0.1.0";
-  # Narrow src (just docs/) keeps this derivation's input hash decoupled
-  # from the rest of the tree — a doc edit only re-hashes this.
-  src = ../docs;
-
-  # No build: pure markdown, nothing to compile or render.
-  dontBuild = true;
-  dontConfigure = true;
-
-  installPhase = ''
-    runHook preInstall
-    mkdir -p $out
-    cp -r ./* $out/
-    runHook postInstall
-  '';
-
-  dontFixup = true;
-
-  meta = {
-    description = "hyperhive reference docs (the repo docs/ tree)";
-    homepage = "https://forge.darkest.space/hyperhive/hyperhive";
-  };
-}