fix(bin): gstack-session-update — detached updater owns its stdio (SIGPIPE)

The backgrounded update subshell inherited the session hook's stdout/stderr
pipes. Once the hook exits and the caller closes them, any child that writes
— git pull's autostash notice, setup output — dies of SIGPIPE, logged as
PULL_FAILED exit=141 with an empty stderr capture (observed in the free-tests
container, and reachable by any production hook runner that closes stdio
promptly). Redirect the fork to /dev/null; all observability already flows
through the session-update log file.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bin/gstack-session-update

@@ -146,6 +146,11 @@ fi
   else
     log_entry "UP_TO_DATE head=$OLD_HEAD"
   fi
-) &
+# The detached subshell must own its stdio: it inherits the session hook's
+# pipes, and once the hook exits and the caller closes them, any child that
+# writes (git pull's autostash notice, setup output) dies of SIGPIPE —
+# observed as PULL_FAILED exit=141 with an empty stderr capture. All
+# observability goes through LOG_FILE.
+) >/dev/null 2>&1 &
 
 exit 0