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);
+}

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);
+        }
+    }
+}

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);
+}

tests/ClaudeDo.Installer.Tests/EnvironmentCheckServiceTests.cs

@@ -0,0 +1,122 @@
+using ClaudeDo.Installer.Checks;
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Tests;
+
+public sealed class EnvironmentCheckServiceTests
+{
+    private sealed class FakeCheck : IEnvironmentCheck
+    {
+        private readonly Func<InstallContext, CancellationToken, Task<CheckResult>>? _run;
+
+        public FakeCheck(string id, CheckSeverity severity, Func<InstallContext, CancellationToken, Task<CheckResult>>? run = null)
+        {
+            Id = id;
+            Severity = severity;
+            _run = run;
+        }
+
+        public string Id { get; }
+        public CheckSeverity Severity { get; }
+
+        public Task<CheckResult> RunAsync(InstallContext ctx, CancellationToken ct) =>
+            _run is null
+                ? Task.FromResult(CheckResult.Ok(Id, Severity, "title", "ok"))
+                : _run(ctx, ct);
+    }
+
+    private sealed class ThrowingCheck : IEnvironmentCheck
+    {
+        public string Id => "throwing";
+        public CheckSeverity Severity => CheckSeverity.Error;
+
+        public Task<CheckResult> RunAsync(InstallContext ctx, CancellationToken ct) =>
+            throw new InvalidOperationException("boom");
+    }
+
+    [Fact]
+    public async Task RunAllAsync_PreservesInputOrder_RegardlessOfCompletionOrder()
+    {
+        var slow = new FakeCheck("slow", CheckSeverity.Warning, async (_, ct) =>
+        {
+            await Task.Delay(50, ct);
+            return CheckResult.Ok("slow", CheckSeverity.Warning, "title", "ok");
+        });
+        var fast = new FakeCheck("fast", CheckSeverity.Warning);
+        var service = new EnvironmentCheckService(new IEnvironmentCheck[] { slow, fast });
+
+        var report = await service.RunAllAsync(new InstallContext(), CancellationToken.None);
+
+        Assert.Equal(new[] { "slow", "fast" }, report.Results.Select(r => r.Id));
+    }
+
+    [Fact]
+    public async Task HasBlockingError_True_WhenErrorAndFailed()
+    {
+        var check = new FakeCheck("id", CheckSeverity.Error, (_, _) =>
+            Task.FromResult(CheckResult.Fail("id", CheckSeverity.Error, "title", "bad")));
+        var service = new EnvironmentCheckService(new IEnvironmentCheck[] { check });
+
+        var report = await service.RunAllAsync(new InstallContext(), CancellationToken.None);
+
+        Assert.True(report.HasBlockingError);
+    }
+
+    [Fact]
+    public async Task HasBlockingError_False_WhenWarningAndFailed()
+    {
+        var check = new FakeCheck("id", CheckSeverity.Warning, (_, _) =>
+            Task.FromResult(CheckResult.Fail("id", CheckSeverity.Warning, "title", "bad")));
+        var service = new EnvironmentCheckService(new IEnvironmentCheck[] { check });
+
+        var report = await service.RunAllAsync(new InstallContext(), CancellationToken.None);
+
+        Assert.False(report.HasBlockingError);
+    }
+
+    [Fact]
+    public async Task HasBlockingError_False_WhenErrorAndUnknown()
+    {
+        var check = new FakeCheck("id", CheckSeverity.Error, (_, _) =>
+            Task.FromResult(CheckResult.Unknown("id", CheckSeverity.Error, "title", "dunno")));
+        var service = new EnvironmentCheckService(new IEnvironmentCheck[] { check });
+
+        var report = await service.RunAllAsync(new InstallContext(), CancellationToken.None);
+
+        Assert.False(report.HasBlockingError);
+    }
+
+    [Fact]
+    public async Task RunAllAsync_ThrowingCheck_BecomesUnknown_OthersStillRun_ServiceDoesNotThrow()
+    {
+        var throwing = new ThrowingCheck();
+        var ok = new FakeCheck("ok", CheckSeverity.Warning);
+        var service = new EnvironmentCheckService(new IEnvironmentCheck[] { throwing, ok });
+
+        var report = await service.RunAllAsync(new InstallContext(), CancellationToken.None);
+
+        var throwingResult = report.Results[0];
+        Assert.Equal(CheckStatus.Unknown, throwingResult.Status);
+        Assert.Contains("boom", throwingResult.Detail);
+
+        var okResult = report.Results[1];
+        Assert.Equal(CheckStatus.Ok, okResult.Status);
+    }
+
+    [Fact]
+    public async Task RunAllAsync_PassesCancellationToken_ToChecks()
+    {
+        CancellationToken? seen = null;
+        var check = new FakeCheck("id", CheckSeverity.Warning, (_, ct) =>
+        {
+            seen = ct;
+            return Task.FromResult(CheckResult.Ok("id", CheckSeverity.Warning, "title", "ok"));
+        });
+        var service = new EnvironmentCheckService(new IEnvironmentCheck[] { check });
+        using var cts = new CancellationTokenSource();
+
+        await service.RunAllAsync(new InstallContext(), cts.Token);
+
+        Assert.Equal(cts.Token, seen);
+    }
+}