Polish impact review flow

[AusafMo]

Summary

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

Tests

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