fix: restore #[tokio::main] on main(), #[cfg(unix)] on platform tests, #[allow] on run_pty_session/AuthListenerResult

The biggest miss is #[tokio::main] — without it, async fn main() doesn't compile,
and the binary in every worktree fails 'cargo check'. Agents in those worktrees
burn their turn budgets trying to fix the build before they can do real work, then
get killed by the watchdog. That's why all three in-flight stories failed.

Other restored attributes:
- #[cfg(unix)] on 4 tests in merge/squash and scaffold (skip on non-Unix)
- #[allow(dead_code)] on AuthListenerResult test enum
- #[allow(clippy::too_many_arguments)] on run_pty_session

Same root cause as the earlier #[test] attribute losses: my line ranges started
at the fn line, missing the leading attribute on the previous line.

server/src/main.rs

@@ -50,6 +50,7 @@ mod cli;
 
 use cli::{CliArgs, parse_cli_args, print_help, resolve_path_arg};
 
+#[tokio::main]
 async fn main() -> Result<(), std::io::Error> {
     // Reap zombie grandchildren on Unix (for native deployments without tini/init).
     // Docker containers with `init: true` in docker-compose.yml already have tini