Add default-off multi-subagent feature switch

[huangruiteng]

Summary

• add a default-off --multi-subagent-feature off|enabled configure-goal switch
• route dashboard/API writes through the feature switch instead of raw orchestration/spawn toggles
• make off a hard single-agent state by clearing child limits/domains, with focused smoke coverage

Validation

• python3 -m py_compile loopx/configure_goal.py loopx/status_server.py loopx/cli_commands/registry_admin.py
• python3 examples/project/configure-goal-smoke.py
• python3 examples/control_plane/control-plane-configure-api-smoke.py
• npm --prefix apps/presentation/dashboard run build
• npm --prefix apps/presentation/dashboard run smoke:home-browser
• git diff --check origin/main...HEAD
• python3 examples/control_plane/check-public-boundary-smoke.py
• PYTHONPATH=. python3 -m loopx.cli canary premerge --from-git-diff --format json --timeout-seconds 60 --no-progress

Notes

• Dashboard build emits the existing Vite chunk-size warning only.
• No self-merge: this touches goal configuration/runtime API and spawn feature policy, so it is left for PR review.