[sosidudku1]

What

Adds POST /api/sessions/{id}/rollback-last-turn. It removes the chat's last question and its answer and returns 200 { id, text } with the question's text. The client then sends text again through /v1/chat/completions, so a retry runs the same path as a first try. Before this, "retry" in every client resent the question on top of the old pair: the chat showed it twice and the model read its first answer before answering again.

• src/session/session-truncate.ts: truncateSessionAt(state, index) cuts a chat back to an earlier row and fixes every field that remembers a transcript position: macroTurnStarts, turnCount, the held conversationPackStart, rollingSummary, the cloud context mirror (trimmed so the next extension does not throw), contextUsage, lastError, status back to pending. A compaction checkpoint that covers the cut is reported, not repaired. No tool policy here, so /rewind and editing a sent message can reuse it.
• planLastTurnRollback(state, { expectText? }) allows it only for a turn that changed nothing outside the transcript: a question answered by one reply, or by nothing (a provider error). A stopped turn is refused (turn_stopped): a stop can cut a tool batch short, and a local turn stores no trace of that step, so a resend could run the tool twice.
• Refusals are 409 with a code: turn_in_progress, turn_has_tools, turn_steered, turn_compacted, turn_stopped, nothing_to_undo, turn_unrecognized, turn_mismatch. expectText and expectPriorQuestions name the question the client means, by its text and by how many questions come before it (steers not counted). If the stored last question differs in either (for example a failed "continue" that never reached the store, after an earlier "continue"), the answer is turn_mismatch and nothing is removed. An unknown chat is 404 session_not_found. An agent without the route answers 404 with no code, which is how a client tells the two apart.
• Before writing, the route stops a compaction running on the chat and clears the turn-progress marker of a stopped turn.

Why

It is the base for a real Regenerate in the desktop and a real [try again] in the terminal. Tool turns are refused on purpose: their effects stay, so a resend would not be a fresh try of the same question. Limits, also written in src/http/README.md: what the turn's reflection already wrote to long-term memory stays, and a chat cut back to nothing keeps its title.

Related PRs: #1164 (terminal retry, stacked on this branch) and #1165 (desktop bridge and buttons, on rel/xp-build).

How it was verified

• New unit tests: src/session/session-truncate.test.ts, src/http/route-rollback-last-turn.test.ts.
• An adversarial code review ran on the diff, and its findings are fixed: the status after a cut, the turn count for a turn that never closed, and comments.
• Lint, tsc and vitest have not been run yet.

Refs ATO-1297, ATO-1298

Not yet checked on the second Mac.