Polish demo impact and PR flow

[AusafMo]

Summary

• let the UI run LLM impact narration through config or demo env overrides instead of a hardcoded browser provider
• render single-record branch diffs as the same side-by-side paper diff used elsewhere and show existing open PR metadata
• make fenced LLM JSON parsing more robust and add a YC demo runbook

Tests

• .venv/bin/ruff check src tests plugins/cfg_impact/cfg_impact
• .venv/bin/python -m pytest

[AusafMo]

Closing in favor of #12 with the neutral branch name codex/impact-review-flow.