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/EnvironmentCheckService.cs

@@ -0,0 +1,37 @@
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Checks;
+
+public sealed record EnvironmentCheckReport(IReadOnlyList<CheckResult> Results)
+{
+    public bool HasBlockingError => Results.Any(r => r.Severity == CheckSeverity.Error && r.Status == CheckStatus.Failed);
+}
+
+public sealed class EnvironmentCheckService
+{
+    private readonly IReadOnlyList<IEnvironmentCheck> _checks;
+
+    public EnvironmentCheckService(IEnumerable<IEnvironmentCheck> checks)
+    {
+        _checks = checks.ToList();
+    }
+
+    public async Task<EnvironmentCheckReport> RunAllAsync(InstallContext ctx, CancellationToken ct)
+    {
+        var tasks = _checks.Select(check => RunSafeAsync(check, ctx, ct)).ToArray();
+        var results = await Task.WhenAll(tasks);
+        return new EnvironmentCheckReport(results);
+    }
+
+    private static async Task<CheckResult> RunSafeAsync(IEnvironmentCheck check, InstallContext ctx, CancellationToken ct)
+    {
+        try
+        {
+            return await check.RunAsync(ctx, ct);
+        }
+        catch (Exception ex)
+        {
+            return CheckResult.Unknown(check.Id, check.Severity, string.Empty, ex.Message, ex.Message);
+        }
+    }
+}