chore: switch SessionStart hook to async mode

Run pnpm install + build in background so sessions start faster.
Timeout set to 5 minutes.

https://claude.ai/code/session_0153jtsmYXCuyedMWwv88aWP

Author: claude

.claude/hooks/session-start.sh

@@ -6,6 +6,9 @@ if [ "${CLAUDE_CODE_REMOTE:-}" != "true" ]; then
   exit 0
 fi
 
+# Run asynchronously (in background while session starts)
+echo '{"async": true, "asyncTimeout": 300000}'
+
 cd "$CLAUDE_PROJECT_DIR"
 
 # Install all workspace dependencies