chore: keep past manual tests as a journeys backlog

[ovitrif]

Description

Merged PRs carry manual tests in description texts that nobody will scan PR history for once journeys replace them. Keep them in the repo so they can be turned into journeys later.

• Add a one-off script that lists merged PRs, extracts #### Manual Tests from each body, and writes journeys/backlog.md grouped by PR with the steps verbatim.
• Run it once and commit the result; a later retro turns entries into journeys and deletes them from the backlog.

Out of Scope

• journeys/: writing the journeys themselves; the backlog only preserves the steps.
• Re-running the script on a schedule; PRs opened after chore: use journeys as prs qa contract #1249 lands list journeys instead.