feat: add managed daemon mode for tunn

[strandnerd]

Summary

• introduce a background daemon that supervises all configured tunnels and exposes an IPC socket for CLI commands
• add tunn start -d to spawn the daemon, write PID/socket/log files under XDG runtime (or ~/.cache/tunn), and validate startup health before returning
• add tunn status to query the daemon, rendering a live ANSI dashboard in terminals or plain text for scripts
• add tunn stop to request a graceful daemon shutdown, waiting briefly for completion
• reuse the existing tunnel manager with a new notifier hook so both foreground and daemon modes share runtime logic and per-port status tracking
• document the new workflow (daemon lifecycle commands, single dependency on gopkg.in/yaml.v3, SSH-based execution, supported platforms) and add an AGENTS.md contributor guide