fix(agent-core-v2): keep background questions open past turn end

[elkaix]

Related Issue

No tracked issue — internal reconciliation pass. Stacked on #307; review that one first (and #306 under it).

Problem

A background AskUserQuestion (asked with background: true) was bound to the asking turn: the moment the agent finished its turn, the question was cancelled and the user never got to answer it. When a background question was answered, the agent received only the path of a saved output file, not the answer itself.

What changed

• Detached question requests no longer carry the asking turn's id, and turn-end cancellation leaves them pending; the question stays open until the user answers or dismisses it.
• The answer is now delivered to the agent inline in the task notification — bounded to 16 KB and XML-escaped inside an <answer> block — instead of only a saved output path. Answered and dismissed outcomes get distinct notification text, and the transcript projection keeps background-question answers grouped with their turn.
• docs/reference/tools.md documents the background behaviour on AskUserQuestion; two patch changesets included.
• dist-web manifest restaged in the same commit (packages/transcript is a web build input; only .web-bundle-manifest.json changed).

Verification on this commit: full pre-push gate (597s, all checks passed), agent-core-v2 suite green, typecheck 0, lint 0, leak-check A–G PASS.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue (external PRs: the issue must have a maintainer's /approve).
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

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📥 Commits

Reviewing files that changed from the base of the PR and between 49548fb and d1d880a.

📒 Files selected for processing (15)

• .changeset/background-question-inline-answer.md
• .changeset/background-question-survives-turn-end.md
• apps/pythinker-code/dist-web/.web-bundle-manifest.json
• docs/reference/tools.md
• packages/agent-core-v2/src/agent/task/taskService.ts
• packages/agent-core-v2/src/agent/tools/ask-user-question/askUserQuestionTool.ts
• packages/agent-core-v2/src/agent/tools/ask-user-question/question-background-task.ts
• packages/agent-core-v2/src/features/interaction/interaction.ts
• packages/agent-core-v2/src/session/question/question.ts
• packages/agent-core-v2/src/session/question/questionService.ts
• packages/agent-core-v2/test/agent/questionTools/tools/ask-user.test.ts
• packages/agent-core-v2/test/agent/task/rpc-events.test.ts
• packages/agent-core-v2/test/session/question/question.test.ts
• packages/transcript/src/history/groupTurns.ts
• packages/transcript/test/layers.test.ts

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[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@pymodel/pythinker-code@09872d7

npx https://pkg.pr.new/@pymodel/pythinker-code@09872d7

commit: 09872d7