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>

.swift-format

@@ -0,0 +1,28 @@
+{
+  "version": 1,
+  "lineLength": 100,
+  "indentation": {
+    "spaces": 4
+  },
+  "tabWidth": 4,
+  "maximumBlankLines": 1,
+  "respectsExistingLineBreaks": true,
+  "lineBreakBeforeControlFlowKeywords": false,
+  "lineBreakBeforeEachArgument": false,
+  "lineBreakBeforeEachGenericRequirement": false,
+  "lineBreakBetweenDeclarationAttributes": false,
+  "prioritizeKeepingFunctionOutputTogether": false,
+  "indentConditionalCompilationBlocks": true,
+  "indentSwitchCaseLabels": false,
+  "spacesAroundRangeFormationOperators": false,
+  "spacesBeforeEndOfLineComments": 2,
+  "indentBlankLines": false,
+  "multiElementCollectionTrailingCommas": true,
+  "reflowMultilineStringLiterals": "never",
+  "fileScopedDeclarationPrivacy": {
+    "accessLevel": "private"
+  },
+  "orderedImports": {
+    "includeConditionalImports": false
+  }
+}