SPIKE swap signald for signal-cli

See: https://git.callpipe.com/finn/signald

h/t to @zig for the find! :)

Questions:

1. Does signald support running multiple simultaneous channels ("usernames") at the same time?
2. What is the interface for interacting with signald via

• how does one listen for events?
• how does one send a message?
• what if the unix socket closes? are we responsible for re-opening?

3. How much memory is consumed by signald per running channel? (less than 150mb beats current impl!)