Install/uninstall commands are macOS LaunchAgent only

[CalebisGross]

Problem

cmd/mnemonic/main.go lines 660-807:

• installCommand() creates a macOS LaunchAgent plist in ~/Library/LaunchAgents/
• uninstallCommand() removes it via launchctl unload

No equivalent exists for Linux (systemd) or Windows (Task Scheduler/Windows Service).

Impact

mnemonic install and mnemonic uninstall only work on macOS.

Fix

Create platform-specific install implementations:

• Linux: Generate and install a systemd user unit file (~/.config/systemd/user/mnemonic.service)
• Windows: Future — Windows Service or Task Scheduler

[CalebisGross]

Resolved by #19 (platform-abstracted ServiceManager with Linux systemd support) and #20 (log directory fix for fresh installs). All CLI commands now work on Linux: install, uninstall, start, stop, restart, serve.