Allow multi-turn conversations inside /ask

Describe the feature or problem you'd like to solve

The /ask command isolates a question from the main conversation, but it only supports one turn. When Copilot asks for clarification, the user cannot answer within the same /ask context.

Proposed solution

Keep /ask isolated from the main conversation while allowing multiple turns inside the same /ask view.

1. Keep the input available after each response.
2. Include previous /ask messages when processing follow-up questions.
3. Preserve this local context until the user closes the /ask view.
4. Let Esc close the side conversation and return to the main session.
5. Do not add the side conversation to the main session unless the user explicitly chooses to do so.

This preserves the purpose of /ask, which is avoiding changes to the primary conversation, while allowing users to clarify questions and reach a useful answer.

Example prompts or workflows

1. Run /ask Why does this API require a cancellation token?. Copilot asks which runtime or API version is being used. Answer inside the same /ask view, then receive a response that uses both messages.
2. Run /ask Compare these two implementation approaches. Copilot asks which constraint matters most. Supply the constraint without adding either message to the main session.
3. Run /ask Explain this error. Copilot asks for a missing command or version. Provide it, receive the explanation, then press Esc to return to the unchanged main session.

Additional context

This replaces #4538 and #4577, which were submitted without the Feature Request form.

Related requests:

1. #92 requests a broader conversation mode that does not perform agent actions.
2. #2885 documents the current dead end when /ask produces a response that requires follow-up interaction.

Start at the /ask command entry point and reproduce a clarification flow with a follow-up question. Trace how the side view handles input and context, then verify that prior /ask messages persist until Esc, remain isolated from the main session, and support multiple turns.