codex: let the agent ask the user a question

[tdwd]

item/tool/requestUserInput is codex's analogue of claude's AskUserQuestion.
It was answered with -32601, so codex could not ask the user anything — the
picker existed but nothing on that backend could reach it.

The request and response shapes come from ToolRequestUserInputParams /
ToolRequestUserInputResponse in the app-server generated schema (codex-cli
0.153.4), not from a probe. The method name and its params type were both
confirmed against ServerRequest.json.

What the two backends do differently

• codex keys each answer by a question id, claude answers positionally. So
  the id rides out on askQuestion.ID and back on approvalReply.answers.
  claude has only the deny-message channel and ignores that field.
• A codex question may offer no options, or be marked isSecret. The picker
  chooses between options and cannot take typed input, so those questions are
  left out of the answer map — which is how codex reads "unanswered", the same
  as a dismissed question. A line says why, because a question that never
  appears reads as a bug in the pane rather than a limit of it.
• isOther is narrowed away for the same reason, matching what the picker
  already does with claude's questions. isBlocking is deliberately unread:
  answering is correct either way, and it is not answering that stops a turn.

One refactor

Every path to the approval pane now goes through codexEngine.ask. It owns the
two rules that hang a turn when one is missed — the wait runs off the reader
goroutine, and one request holds the pane at a time. askUser had both inline,
so a third request kind was free to duplicate them or forget them.

Tests

Five new tests. Three were confirmed discriminating by actually reverting the
code they cover:

• removing the routing case → TestCodexQuestionRoundTrip times out with no
  answer on the wire, which is what a hung turn looks like;
• dropping answers: from the reply → TestAnsweringSendsBothFormsOfTheAnswer
  fails with {"answers":{}};
• moving the build-mode short-circuit above the question check →
  TestCodexQuestionIsAskedEvenInBuildMode fails.

go build, go vet and the full suite are green.

Not verified: this has not been driven against the live codex CLI. Getting
a real agent to ask a question on demand needs a turn, and plan item 1.7 (a
human at a terminal) is still open.