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/Core/InstallContext.cs

@@ -32,4 +32,8 @@ public sealed class InstallContext
 
     // InstallPage
     public bool CreateDesktopShortcut { get; set; } = true;
+
+    // WelcomePage — register the external MCP endpoint with the Claude CLI.
+    public bool RegisterMcpWithClaude { get; set; } = true;
+    public int ExternalMcpPort { get; set; } = 47_822;
 }