agentd: Implement agent start/stop/restart via systemd

Add start/stop/restart commands to the agentd agent CLI group.

Commands: 1. agentd agent start <name>

• Verify config exists in DB/env file
• Run systemctl --user start pi-agent@<name>
• Update DB status to 'running'
• Print status confirmation

2. agentd agent stop <name>

• Run systemctl --user stop pi-agent@<name>
• Update DB status to 'stopped'

3. agentd agent restart <name>

• Run systemctl --user restart pi-agent@<name>
• Update DB status to 'running'

Important: must set XDG_RUNTIME_DIR and DBUS_SESSION_BUS_ADDRESS env vars when calling systemctl, same as the current pi-agent script does:

XDG_RUNTIME_DIR=/run/user/$(id -u)
DBUS_SESSION_BUS_ADDRESS=unix:path=$XDG_RUNTIME_DIR/bus

Use System.Process to shell out to systemctl. Check exit codes and report errors clearly.

Files to modify:

• Omni/Agentd.hs (CLI dispatch)
• Omni/Agentd/Daemon.hs (systemd interaction functions)