feat: switch from tmux to simple nohup for remote jobs

- Remove tmux-based execution; jobs now run via nohup + shell script
- Remove p-agent daemon from workspace
- Remove `p attach` command (use `p logs -f` instead)
- Add `-d/--detach` flag to start job without streaming
- Stream output via SSH tail -F
- Ctrl+C detaches from stream; job keeps running
- `p stop` kills via PID file
- Worker requirements reduced to just rsync (no tmux needed)

Cargo.toml

@@ -1,3 +1,3 @@
 [workspace]
-members = ["p", "agent"]
+members = ["p"]
 resolver = "2"