feat: cross-platform support (Windows + Linux)

[espetro]

Make the core server (config, HTTP/MCP layer, Android adapter, agent-device controller) run correctly on Windows and Linux; keep iOS Simulator support macOS-only (inherently requires Xcode); correct docs; add a native cross-platform service manager.

What's included

• platforms/android — dropped darwin/linux-only build constraint; extracted setProcAttr into procattr_unix.go/procattr_windows.go (Setpgid vs CREATE_NEW_PROCESS_GROUP); portable Android SDK home-dir probing (macOS/Linux/Windows defaults via %LOCALAPPDATA%)
• controllers/agentdevice — replaced Unix-only pkill -f with cmd.Process.Kill() (cross-platform stdlib)
• internal/config — replaced raw $HOME env lookup with os.UserHomeDir() for portable default config path
• .github/workflows/ci.yml — new cross-platform matrix job (ubuntu/windows/macos) running build+vet+test
• docs/adding-platform.md — updated Setpgid guidance to point at the new procattr pattern
• README.md / AGENTS.md — corrected wording; only iOS needs macOS, rest of the server runs everywhere
• New mcp-sim service subcommand (install/uninstall/start/stop/restart/status/run) via github.com/kardianos/service — installs mcp-sim as a native background service (launchd/systemd/Windows Service), including a --user mode for non-root installs on macOS/Linux
• docs/launchd.md → docs/service.md

Verification

• go build ./..., go vet ./..., task validate all green
• Cross-compiled clean for GOOS=windows and GOOS=linux
• Manual smoke test: mcp-sim serve boots iOS+Android+agentdevice adapters, /healthz reachable, graceful shutdown on signal
• Manual smoke test: mcp-sim service install --user/start/status/stop/uninstall full cycle on macOS — install/uninstall lifecycle confirmed working; launchctl load for user LaunchAgents needs an active GUI login session (works from a real Terminal session, not from this sandboxed shell)