feat(installer): add check abstraction and EnvironmentCheckService

Scaffolding for environment checks: IEnvironmentCheck, CheckResult/
CheckSeverity/CheckStatus, and EnvironmentCheckService that runs checks
in parallel while preserving input order, never throws (a failing
check becomes Unknown), and reports HasBlockingError only for
Error+Failed.

src/ClaudeDo.Installer/Checks/CheckResult.cs

@@ -0,0 +1,25 @@
+namespace ClaudeDo.Installer.Checks;
+
+public enum CheckSeverity { Warning, Error }
+
+public enum CheckStatus { Ok, Failed, Unknown }
+
+public sealed record CheckResult(
+    string Id,
+    CheckSeverity Severity,
+    CheckStatus Status,
+    string TitleKey,
+    string Message,
+    string? HintKey,
+    string? HelpUrl,
+    string? Detail)
+{
+    public static CheckResult Ok(string id, CheckSeverity severity, string titleKey, string message, string? detail = null) =>
+        new(id, severity, CheckStatus.Ok, titleKey, message, HintKey: null, HelpUrl: null, detail);
+
+    public static CheckResult Fail(string id, CheckSeverity severity, string titleKey, string message, string? hintKey = null, string? helpUrl = null, string? detail = null) =>
+        new(id, severity, CheckStatus.Failed, titleKey, message, hintKey, helpUrl, detail);
+
+    public static CheckResult Unknown(string id, CheckSeverity severity, string titleKey, string message, string? detail = null) =>
+        new(id, severity, CheckStatus.Unknown, titleKey, message, HintKey: null, HelpUrl: null, detail);
+}