fix(app): derive shutdown context from context.Background() instead of cancelled globalCtx

[AnyCPU]

• I have read CONTRIBUTING.md.
• I have created a discussion that was approved by a maintainer (for new features).

When the user presses Ctrl+C, signal.NotifyContext cancels globalCtx. Shutdown() then derives shutdownCtx from it.

A child of a cancelled context is born cancelled, so the 5-second timeout never takes effect. All context-dependent cleanup silently short-circuits:

• shell.KillAll(shutdownCtx) — returns immediately, background shells not waited on
• mcp.Close(shutdownCtx) — returns immediately, MCP sessions not closed

Fix

Use context.WithTimeout(context.Background(), 5*time.Second) so shutdown cleanup gets a live 5-second window.

[AnyCPU]

@caarlos0 could you by any chance take a look at this mr :-) a fix is simple but useful.