Add daemon supervisor with health monitoring

[bug-ops]

Epic: #367

Description

Background process supervisor managing agent, gateway, channels with health checks and graceful restart.

Tasks

• zeph --daemon flag to run as background process
• Component supervisor: agent loop, gateway, active channel
• Health monitoring with configurable interval
• Exponential backoff on component failure
• PID file at ~/.zeph/zeph.pid
• zeph --stop sends SIGTERM to daemon
• Graceful shutdown propagates to all components (reuse existing Ctrl-C handler)