Require reading DMs before sending to prevent context-free messages
read-dms.sh now records a read timestamp to ~/.claude-social/dms-last-read.json
after fetching messages. send-dm.sh checks this file before sending and refuses
if the conversation wasn't read within the last 10 minutes, with a clear error
message to re-read first.
This mirrors how chat UIs work: you see prior messages before composing a reply.