ci(scenarios): nightly E2E pass for Glass Box scenario registry against live backends

[roryford]

Context

Glass Box P3/P4 scenarios run hermetically in CI (scripted mode via ScriptedGenerationBackend). The #1531 spec called for a nightly live-backend pass that exercises the same RuntimeScenarioRegistry entries against real inference backends — asserting only the structural event subsequence, not token content.

What's needed

• A nightly CI job (.github/workflows/nightly.yml or added to the existing nightly tier) that runs test_allRegisteredScenarios_passInLiveMode against Ollama (already in the nightly trait set) and optionally CloudSaaS
• The live-mode test method in ManifoldRuntimeTests (the scripted-mode gate test_allRegisteredScenarios_passInScriptedMode already exists; live mode needs a parallel method guarded by --traits Ollama)
• Failure notifications distinct from the per-PR gate so a flaky live model doesn't block PRs

Scope

• Ollama is the default live target (local model, deterministic enough for structural asserts)
• CloudSaaS gated separately (requires env secrets); optional in v1
• Each scenario's structural [ConversationEventKind] subsequence is the assertion oracle — same data the scripted gate uses

Policy

Add to the existing nightly workflow tier rather than creating a new workflow file.