[codex] Add first-class Sora video provider

[calesthio]

Summary

• Add sora_video as a first-class OpenMontage video_generation provider for OpenAI Sora.
• Route Sora availability through the normal BaseTool / registry dotenv path, so OPENAI_API_KEY from repo .env is discoverable during preflight.
• Add regression coverage for registry discovery and repo .env loading from a clean process environment.

Root Cause

Sora was only available through the standalone Codex skill CLI. That CLI checks the already-running process environment and does not load OpenMontage's repo .env, while OpenMontage tools and registry do load .env via BaseTool / ToolRegistry.

Validation

• python -m pytest tests\tools\test_sora_video.py -q
• python -m pytest tests\tools\test_sora_video.py tests\contracts\test_phase1_contracts.py tests\contracts\test_phase2_contracts.py tests\tools\test_hyperframes_compose.py::test_provider_menu_summary_returns_expected_shape -q
• git diff --check
• python -m py_compile tools\video\sora_video.py tests\tools\test_sora_video.py