Add beads and gastown to CI cache, consolidate CI workflow

- Add packages/beads and packages/gastown with shared definitions
- Expose custom-beads and custom-gastown in flake packages output
- Consolidate CI from matrix (8 parallel jobs) to single job with loop
- Saves ~12 minutes of redundant nix-setup time per run
- Uses ::group:: for collapsible log sections per package

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/beads/default.nix

@@ -0,0 +1,41 @@
+# Beads package - issue tracker for AI-supervised coding workflows
+# Takes src as argument so it can be called from both overlay and flake packages
+{ lib
+, stdenv
+, buildGoModule
+, fetchurl
+, go_1_25
+, git
+, pkg-config
+, icu
+, src
+, version ? "unknown"
+}:
+
+let
+  # nixpkgs ships Go 1.25.5, but beads' dolt deps require Go >= 1.25.6
+  go_1_25_6 = go_1_25.overrideAttrs (old: rec {
+    version = "1.25.6";
+    src = fetchurl {
+      url = "https://go.dev/dl/go${version}.src.tar.gz";
+      hash = "sha256-WMv3ceRNdt5vVtGeM7d9dFoeSJNAkih15GWFuXXCsFk=";
+    };
+  });
+  buildGoModule_1_25_6 = buildGoModule.override { go = go_1_25_6; };
+in
+buildGoModule_1_25_6 {
+  pname = "beads";
+  inherit version src;
+  subPackages = [ "cmd/bd" ];
+  doCheck = false;
+  # Regenerated vendorHash for commit 6a51223b (dolt server mode, Go 1.25.6)
+  vendorHash = "sha256-9RMy0+ZBFg1BAl8Z0EuZK4XVm9QYVekS9i/1ErOIB/c=";
+  nativeBuildInputs = [ git pkg-config ];
+  buildInputs = [ icu ];
+  meta = with lib; {
+    description = "beads (bd) - An issue tracker designed for AI-supervised coding workflows";
+    homepage = "https://github.com/steveyegge/beads";
+    license = licenses.mit;
+    mainProgram = "bd";
+  };
+}