feat(installer): optionally register ClaudeDo MCP server with Claude

Add an install step and welcome-page opt-in that registers the ClaudeDo
external MCP server with the Claude CLI. Failures are non-fatal and surface
the manual command so a missing or old CLI never blocks the install.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ClaudeDo.Installer/Steps/RegisterMcpStep.cs

@@ -0,0 +1,47 @@
+using ClaudeDo.Installer.Core;
+
+namespace ClaudeDo.Installer.Steps;
+
+public sealed class RegisterMcpStep : IInstallStep
+{
+    private const string ServerName = "claudedo";
+
+    public string Name => "Register MCP with Claude";
+
+    public async Task<StepResult> ExecuteAsync(InstallContext ctx, IProgress<string> progress, CancellationToken ct)
+    {
+        if (!ctx.RegisterMcpWithClaude)
+        {
+            progress.Report("Skipped (not selected)");
+            return StepResult.Ok();
+        }
+
+        var url = $"http://127.0.0.1:{ctx.ExternalMcpPort}/mcp";
+
+        // Drop any prior registration first so a re-run (e.g. update, changed port)
+        // overwrites cleanly instead of erroring on a duplicate name.
+        progress.Report($"Removing existing '{ServerName}' MCP registration (if any)...");
+        await ProcessRunner.RunAsync(ctx.ClaudeBin, $"mcp remove --scope user {ServerName}", null, progress, ct);
+
+        progress.Report($"Registering '{ServerName}' MCP server at {url}...");
+        var (exit, output) = await ProcessRunner.RunAsync(
+            ctx.ClaudeBin,
+            $"mcp add --transport http --scope user {ServerName} {url}",
+            null, progress, ct);
+
+        // Non-fatal: a missing/old Claude CLI must never block the install. Surface the
+        // manual command so the user can register it themselves later.
+        if (exit != 0)
+        {
+            progress.Report(
+                $"Could not register MCP automatically (claude exited {exit}). " +
+                $"Run manually: claude mcp add --transport http --scope user {ServerName} {url}");
+        }
+        else
+        {
+            progress.Report("MCP server registered with Claude.");
+        }
+
+        return StepResult.Ok();
+    }
+}