feat(nix): saturate builds, add community cache, pin nixpkgs registry

- Drop auto-optimise-store: slows every build for modest disk savings.
  Run 'nix store optimise' manually if disk pressure ever shows up.
- max-jobs=auto, cores=0: defaults are 1/1, which left most of the box
  idle during large closures (LLVM, protobuf, …).
- Add nix-community.cachix.org as an extra substituter with its public
  key. Big hit-rate boost against nixos-unstable, which is what the new
  user registry points 'nixpkgs' at.
- dot_config/nix/registry.json pins 'nixpkgs' indirect ref to
  github:NixOS/nixpkgs/nixos-unstable, so 'nix shell nixpkgs#foo' is
  fast + reproducible. Project flakes are unaffected — they pin their
  own inputs via flake.lock.

1 files changed, 17 insertions, 0 deletions

diff --git a/dot_config/nix/registry.json b/dot_config/nix/registry.json
new file mode 100644
index 0000000..387ea7a
--- /dev/null
+++ b/dot_config/nix/registry.json
@@ -0,0 +1,17 @@
+{
+  "version": 2,
+  "flakes": [
+    {
+      "from": {
+        "type": "indirect",
+        "id": "nixpkgs"
+      },
+      "to": {
+        "type": "github",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "ref": "nixos-unstable"
+      }
+    }
+  ]
+}