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/Interfaces/IEnvironmentCheck.cs

@@ -0,0 +1,10 @@
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Checks;
+
+public interface IEnvironmentCheck
+{
+    string Id { get; }
+    CheckSeverity Severity { get; }
+    Task<CheckResult> RunAsync(InstallContext ctx, CancellationToken ct);
+}