Fix Morph launcher flow and restore top-level Gauss entry

[gauss-math-inc]

Summary

• force Morph template cache rebuilds in the shared-template publisher
• when no main provider is staged, launch Gauss with /start after gauss setup succeeds
• cover the cache-force default and the no-provider launcher handoff in tests

Validation

• python -m pytest tests/test_publish_shared_template.py -q
• bash -n scripts/install-internal.sh
• bash -n tests/installer/ubuntu_repository_local_install_smoke/run-in-container.sh
• fresh temp-home install with no staged provider keys, then PTY smoke showing gauss-launch-session calls gauss setup followed by gauss --startup-input /start