Bulk-update stacked PR bodies from one markdown file with gh pr edit --body-file

When a stack of PRs gets its descriptions rewritten, keep all bodies in one markdown file (one ### heading per PR with key, head and base) and regenerate per-PR body files with a short python split, then loop gh pr edit <num> -R owner/repo --body-file <file>. Map keys to PR numbers once after gh pr create. This keeps the file and GitHub in sync and avoids hand-pasting 10+ bodies.

Lesson from a rewrite pass: when you shorten a claim, re-check it against code. Rewriting "removing a reviewer clears their assignment so they do not get that applicant again" exposed an overclaim — the code only deletes the current assignment if it matches that applicant; nothing blocks a later re-assignment.