[Stability][BUG-006] Chat export includes hidden context, memory, reasoning, and trace

Parent stability dashboard: #3107

This issue tracks BUG-006 from #3107.

Problem

Normal chat export can include content that is not visible in the chat transcript.

Observed exported content included:

• <system-reminder>
• <memory>
• <current_date>
• Thinking/reasoning details
• tool call names / trace-like information

Important distinction: this was not observed as raw system prompt leakage. The concrete issue is that hidden dynamic context, memory, reasoning content, and debug trace can be included in a normal user export.

Code evidence

The chat UI has hidden-message filtering, but export does not appear to apply the same boundary:

frontend/src/core/threads/export.ts

Markdown export includes reasoning blocks:

<details>
<summary>Thinking</summary>
...
</details>

JSON export maps raw messages more directly and can include tool-related fields.

Source: checkpoint/state inspection of AI messages.

AIMessage.additional_kwargs.reasoning_content

So even if the normal UI hides it, export paths must explicitly filter it.

Impact

• A user export is not a clean transcript.
• Memory injected into model context can be exported as if it were part of the conversation.
• Reasoning/tool traces can expose internal behavior that users did not ask to export.
• Product privacy/debug boundaries are ambiguous.

Expected behavior

Default export should include only the user-visible transcript:

• visible user messages;
• visible assistant final answers;
• visible artifact/file references if already shown to the user.

Default export should exclude:

• hidden messages marked hide_from_ui;
• dynamic context reminders;
• memory injection;
• thinking/reasoning content;
• tool calls and tool results.

If raw trace export is needed, it should be a separate explicit debug/admin export surface.