nix: a flake that builds every binary, the tool image and a dev shell

Follows the owner's other projects (sift, tether): nixpkgs pinned to
nixos-26.05, x86_64-linux, the version from the commit. `packages.default`
builds all six binaries with rustPlatform (tests stay in `make gate`),
`packages.tools-image` is deploy/tools-image.nix, and `devShells.default`
has Rust 1.95.0, clippy, rustfmt, cargo-deny and make. No NixOS module
yet: that is M8 (P22).

Checked on straylight: `nix flake check` passes; `nix build` gives
brokerd, bxctl, gatewayd, inferproxy, loopd and toolkit, each printing
its usage; `nix build .#tools-image` and `nix develop` work.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,25 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1790218706,
+        "narHash": "sha256-qdM4Skrqq9Xps3OPNi9PWC6/hrWK9eDx5DUAUUaoz+o=",
+        "rev": "c508844df6c28fa6dabc1b6af70f3ccbd65c5201",
+        "type": "tarball",
+        "url": "https://releases.nixos.org/nixos/26.05/nixos-26.05.10529.c508844df6c2/nixexprs.tar.zst"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixos-26.05",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}