The terminal's pull screen offers Take back on a shipped pull, as the page does #7228

For: undoing a shipped change is the same verb on every surface.

Today a shipped pull can be taken back from the pull page (POST /:owner/:name/pulls/:number/take-back, src/web/studio/actions.ts) and the API (POST /api/v1/repos/:owner/:name/pulls/:number/take-back, src/http/api/take_back.ts, #7220), but src/tui/screens/pull.ts has no key for it.

Done:

• [ ] On a shipped pull, the terminal's pull screen offers Take back (a key named in its footer), which calls revertPull with the signed-in person and opens the new pull's screen.
• [ ] Refusals show in the screen's own words, the same ones the page uses (takeBackWords): not shipped, already taken back, mirrored, an agent refused, a conflict naming the files.
• [ ] A test drives the screen: ship a pull, take it back, and the new pull is Revert "<title>".
• [ ] README's take-back paragraph no longer says the terminal does not offer it.

Risk: low. The rules stay in revertPull.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.