feat: Add Nix flake for declarative builds

Add flake.nix with computed vendorHash for Go dependencies. The flake
supports all major platforms (x86_64/aarch64 Linux and macOS) and
builds the bd binary from cmd/bd.

Removed unnecessary postInstall section as buildGoModule already names
the binary correctly from subPackages.

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

Co-Authored-By: Claude <noreply@anthropic.com>

perf: Use self input instead of ./. for faster Nix evaluation

Replace src = ./. with src = self in flake.nix to reduce unnecessary
file copying during evaluation. This eliminates the Nix warning about
evaluation performance.

flake.lock

@@ -0,0 +1,61 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1760284886,
+        "narHash": "sha256-TK9Kr0BYBQ/1P5kAsnNQhmWWKgmZXwUQr4ZMjCzWf2c=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "cf3f5c4def3c7b5f1fc012b3d839575dbe552d43",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}