Plan

Extend the region slice with the three prompt-marker bases and a top direction (first instead of last lines); prompt detection follows herdr's codex_prompt_line and codex_block_marker_line. Port codex.toml, splitting herdr's nested any/all gates into rules or alternations as in specs 028/030.

Test-first: region unit tests, then Codex fixtures. Record the regions in ADR-0027 and docs/07. A Claude review agent reviews the diff; no Gemini.