feat(brainstorm-server): opt-in auto-open of the browser on the first screen

When the user approves the visual companion, open their browser automatically the
first time a screen is actually ready to show — rather than at startup (just the
waiting page) or making them open the URL by hand.

Opt-in and gated on approval: off unless BRAINSTORM_OPEN is set (start-server.sh
--open, which the agent passes only after the user agrees to use the companion).
Even then it fires once, and is skipped if a browser is already connected, on a
non-loopback/remote bind, or when headless. Launcher is the platform default
(open / xdg-open / WSL cmd.exe) or BRAINSTORM_OPEN_CMD; best-effort, never fatal.

lifecycle.test.js: opens once on the first screen when approved; does NOT open
without approval.

Closes #755
Refs #759

skills/brainstorming/scripts/start-server.sh

@@ -12,6 +12,8 @@
 #                         Use 0.0.0.0 in remote/containerized environments.
 #   --url-host <host>     Hostname shown in returned URL JSON.
 #   --idle-timeout-minutes <n>  Shut down after n minutes idle (default 240 = 4h).
+#   --open                Auto-open the browser on the first screen (use only
+#                         after the user approves the visual companion).
 #   --foreground          Run server in the current terminal (no backgrounding).
 #   --background          Force background mode (overrides Codex auto-foreground).
 
@@ -42,6 +44,10 @@ while [[ $# -gt 0 ]]; do
       IDLE_TIMEOUT_MINUTES="$2"
       shift 2
       ;;
+    --open)
+      export BRAINSTORM_OPEN=1
+      shift
+      ;;
     --foreground|--no-daemon)
       FOREGROUND="true"
       shift