agentd: Implement agent send/logs/rm commands

Add the remaining agent management commands to the agentd agent CLI group.

Commands: 1. agentd agent send <name> <message>

• Write RPC JSON to FIFO at ~/.local/state/pi-agents/<name>.fifo
• Format: {"type": "prompt", "message": "<message>"}
• Record the message in SQLite messages table
• Error if FIFO doesn't exist (agent not running)

2. agentd agent logs <name> [-f]

• Pass through to journalctl --user -u pi-agent@<name> --no-pager
• If -f flag: add --follow

3. agentd agent rm <name>

• Stop the agent if running (systemctl --user stop)
• Delete env file from ~/.config/pi-agents/<name>.env
• Clean up DB records
• Clean up FIFO if it exists

For FIFO writing, use System.Posix.IO or System.IO to open the named pipe for writing. The JSON must be followed by a newline.

Files to modify:

• Omni/Agentd.hs (CLI parsing + dispatch)
• Omni/Agentd/Daemon.hs (FIFO write, journalctl passthrough, cleanup functions)