Wire cloud review caller

[samsamtrum]

Summary

• wires the cloud-review gate to make one LiteLLM chat-completion call per gated file
• saves cloud responses under the dated _cloud output directory and links them from 08_CLOUD_REVIEW_CANDIDATES.md
• adds configurable per-model cloud pricing and budget preflight/actual-cost checks
• updates cloud-review documentation status from scaffolded to implemented

Tests

• .venv/bin/python -m pytest tests/test_cloud_review.py tests/test_execution_steps.py -q

Closes #5