Install macOS helper from PKG for XPC enroll

Browser enrollment used osascript on every elevate. Ship a signed
privileged helper installed at PKG time so probo:// can enroll over
XPC with no second admin prompt. Add make install/uninstall/clean for
local PKG test loops, and show alerts only on failure.

Mirror the Go lint path for the macOS SPM package: Make
targets, root configs, and a Linux CI job. Keep checks
syntax-only so they do not need a macOS SDK. Format the
existing sources so the new gates start clean.

Signed-off-by: Ludovic Vielle <ludovic@probo.com>

cmd/probo-agent/installer/macos/Resources/welcome.html

@@ -48,6 +48,9 @@
         <li>Installs <code>Probo Agent.app</code> to
             <code>/Applications</code> so browser enrollment deep links
             (<code>probo://</code>) work.</li>
+        <li>Installs the privileged helper
+            <code>com.probo.agent.helper</code> so browser enrollment can
+            finish without a second admin password prompt.</li>
         <li>Registers the menu bar helper LaunchAgent
             <code>com.probo.agent.tray</code> in
             <code>/Library/LaunchAgents</code>.</li>