feat(installer): pull in preflight check implementations as prerequisite for SystemCheckPage

Git/GitIdentity/Port/WriteAccess and Claude CLI/Version/Auth/PermissionModeAuto
checks plus the ExecutableResolver they depend on were built in two sibling
task branches that hadn't landed on main yet. Vendored the finished files in
from those branches (same content, verified building + tests green) so the
SystemCheckPage task has something to consume.

tests/ClaudeDo.Installer.Tests/Checks/GitCheckTests.cs

@@ -0,0 +1,66 @@
+using ClaudeDo.Installer.Checks;
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Tests.Checks;
+
+public sealed class GitCheckTests : IDisposable
+{
+    private readonly string _dir;
+
+    public GitCheckTests()
+    {
+        _dir = Path.Combine(Path.GetTempPath(), $"cdgitcheck_{Guid.NewGuid():N}");
+        Directory.CreateDirectory(_dir);
+    }
+
+    public void Dispose()
+    {
+        try { Directory.Delete(_dir, recursive: true); } catch { }
+    }
+
+    private string MakeGitExe()
+    {
+        var path = Path.Combine(_dir, "git.exe");
+        File.WriteAllText(path, "");
+        return path;
+    }
+
+    [Fact]
+    public async Task Git_present_with_parseable_version_reports_ok()
+    {
+        MakeGitExe();
+        var runner = new FakeProcessRunner((0, "git version 2.43.0.windows.1\n"));
+        var check = new GitCheck(runner, pathOverride: _dir, pathExtOverride: ".exe");
+
+        var result = await check.RunAsync(new InstallContext(), CancellationToken.None);
+
+        Assert.Equal(CheckStatus.Ok, result.Status);
+        Assert.Contains("2.43.0", result.Message);
+        Assert.Contains(_dir, result.Message);
+    }
+
+    [Fact]
+    public async Task Git_not_found_on_path_reports_failed()
+    {
+        var runner = new FakeProcessRunner();
+        var check = new GitCheck(runner, pathOverride: _dir, pathExtOverride: ".exe");
+
+        var result = await check.RunAsync(new InstallContext(), CancellationToken.None);
+
+        Assert.Equal(CheckStatus.Failed, result.Status);
+        Assert.Equal(CheckSeverity.Error, result.Severity);
+        Assert.Empty(runner.Calls);
+    }
+
+    [Fact]
+    public async Task Git_nonzero_exit_reports_failed()
+    {
+        MakeGitExe();
+        var runner = new FakeProcessRunner((1, "some error"));
+        var check = new GitCheck(runner, pathOverride: _dir, pathExtOverride: ".exe");
+
+        var result = await check.RunAsync(new InstallContext(), CancellationToken.None);
+
+        Assert.Equal(CheckStatus.Failed, result.Status);
+    }
+}