A hand-over card has "Give back to the crew": one press assigns the pull back to the builder that opened it, and its run starts #6419

Flow work for #5930 (the steward's trap 1, 2026-10-26). When a pass gives up and assigns a pull to the owner, the crew treats it as held by a person. Today the only way back is to reassign by hand: re-assigning #4413 to scout started a run at once.

Done when:

• [ ] Each hand-over card on the board (src/web/pages/board.ts, the cards crew/handover.ts describes) has a "Give back to the crew" button: a POST form with the forge's same-origin check, for the owner or a writer only.
• [ ] Pressing it assigns the pull (and its issue) back to the agent that authored the pull, writes a comment "Given back to the crew by @who", and the card leaves the board. The next tick dispatches it the way any assigned pull is dispatched.
• [ ] A pull whose author is a person, or an agent that no longer exists, gets no button.
• [ ] Tests (a new test file, listed in scripts/gate.ts if that list names test files): the button is drawn on a crew pull's card and not on a person's; the POST reassigns and comments; a foreign Origin is refused; a reader without write access gets 404.

Files: src/web/pages/board.ts, the route that board forms already post to, a new test/handover-give-back.test.ts.