Add macOS support with Homebrew + Xcode CLT bootstrap

Detect Darwin automatically and, on macOS, install the Xcode Command
Line Tools and Homebrew as the first actions before any package work.
Route system packages through brew (formulae and casks), skip the
Flatpak section entirely, refuse to run as root (brew won't), and
adjust custom-package URL/install-path templates with new {os}, {os_go},
{os_zig}, {os_nvim} substitutions. Architecture detection remains
dynamic on both Apple Silicon and Intel.

https://claude.ai/code/session_01R2CnCeEBYT5QoHiwxczDNq

formatted_packages.py

@@ -10,6 +10,10 @@ URL templates use ``str.format`` with the following substitutions:
     {version}   pkg.version (or the latest resolved version)
     {arch}      "x86_64" or "aarch64"
     {arch_go}   Go-style: "amd64" or "arm64"
+    {os}        "linux" or "macos"
+    {os_go}     Go-style: "linux" or "darwin"
+    {os_zig}    Zig-style: "linux" or "macos"
+    {os_nvim}   Neovim-style: "linux" or "macos"
 """
 
 SYSTEM_PACKAGES: list[str] = [
@@ -84,7 +88,7 @@ CUSTOM_PACKAGES: list[dict] = [
     {
         "name": "go",
         "version": "1.26.3",
-        "url_template": "https://go.dev/dl/go{version}.linux-{arch_go}.tar.gz",
+        "url_template": "https://go.dev/dl/go{version}.{os_go}-{arch_go}.tar.gz",
         "sha256": "2b2cfc7148493da5e73981bffbf3353af381d5f93e789c82c79aff64962eb556",
         "fetch_latest": "go",
     },
@@ -102,9 +106,9 @@ CUSTOM_PACKAGES: list[dict] = [
     {
         "name": "zig",
         "version": "0.16.0",
-        "url_template": "https://ziglang.org/download/{version}/zig-{arch}-linux-{version}.tar.xz",
+        "url_template": "https://ziglang.org/download/{version}/zig-{arch}-{os_zig}-{version}.tar.xz",
         "sha256_url_template": (
-            "https://ziglang.org/download/{version}/zig-{arch}-linux-{version}.tar.xz.minisig"
+            "https://ziglang.org/download/{version}/zig-{arch}-{os_zig}-{version}.tar.xz.minisig"
         ),
         "minisign_key": "RWSGOq2NVecA2UPNdBUZykf1CCb147pkmdtYxgb3Ti+JO/wCYvhbAb/U",
         "fetch_latest": "zig",