Daemon uses launchctl unconditionally — crashes on Linux #1

Problem

internal/daemon/daemon.go lines 152-193 call launchctl (macOS-only) with no platform guard:

• IsServiceInstalled() — runs launchctl list
• IsServiceRunning() — parses launchctl list output
• StopService() / StartService() — run launchctl commands

cmd/mnemonic/main.go lines 163-188 call these functions unconditionally in startCommand().

On Linux, launchctl doesn't exist, so these calls fail with "executable file not found".

Impact

mnemonic start crashes on Linux. Users must use mnemonic serve (foreground) as a workaround.

Fix

Add runtime.GOOS checks or use build-tag-separated files:

• internal/daemon/service_darwin.go — existing launchctl code
• internal/daemon/service_linux.go — systemd unit file + systemctl
• internal/daemon/service.go — platform-agnostic interface