test(installer): add tiny managed /prove staging smoke

[gauss-math-inc]

Port of opengauss-dev#24 onto public main.

This adds a tiny Ubuntu installer smoke that:

• installs from the current checkout
• copies a tiny Lean project with one sorry
• initializes a local .gauss manifest
• resolves a managed /prove HelloSorry/Basic.lean run
• verifies the staged skill, startup context, backend instructions, and MCP config all point at the tiny Lean project correctly

Validation on this branch:

• bash -n tests/installer/ubuntu_managed_prove_sorry_smoke/run.sh
• bash -n tests/installer/ubuntu_managed_prove_sorry_smoke/run-in-container.sh
• python -m py_compile tests/installer/ubuntu_managed_prove_sorry_smoke/managed_prove_smoke.py
• deterministic local staging run of managed_prove_smoke.py against the fixture project (without --live-run)

Docker is not available in this devbox, so I did not run the literal container harness here.