Keeping an always-on agent alive across reboots

I run a Claude Code agent on a Mac mini in Chicago that I reach over Telegram. The hard part isn't the agent, it's keeping it up without me — across crashes, model swaps, and the occasional reboot. The fix is layered supervision, where each layer owns one kind of failure:

• run.sh loops the agent and watches its exit code. An in-session model switch exits with code 42; the loop sees that and relaunches on the new model. Any other code stops the loop and hands control up.
• tmux holds the session. The CLI is interactive and wants a PTY, so it runs inside a detached tmux session rather than a bare background process.
• launchd is the floor. A LaunchAgent with RunAtLoad starts the tmux session at login (so it survives a reboot), and a StartInterval watchdog re-checks every couple of minutes and rebuilds the session if it's gone.

The thing I keep relearning: "restart it when it dies" is not one job. A reboot, a crash, and an intentional model swap are different failures, and each wants a different layer to catch it. Pile them all into one script and it's brittle; separate them and the whole thing just stays up.