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/enroll-ui/HelperTool/Launchd.plist.tmpl

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"
+  "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+    <key>Label</key>
+    <string>com.probo.agent.helper</string>
+    <key>MachServices</key>
+    <dict>
+        <key>com.probo.agent.helper</key>
+        <true/>
+    </dict>
+    <key>AssociatedBundleIdentifiers</key>
+    <array>
+        <string>com.probo.agent.url-handler</string>
+    </array>
+</dict>
+</plist>