Pinned items are transmitted to Navie as "code selection" and therefore don't work well as custom prompts

Looking at code in Navie like openai-completion-service:

  async *complete(messages: readonly Message[], options?: CompleteOptions): Completion {
    const tokens = new Array<string>();
    const model = options?.model ?? this.model.modelName;
    const isO1 = this.modelName.startsWith('o1-');
    const usage = new Usage(COST_PER_M_TOKEN[model]);
    let sentMessages: Message[] = mergeSystemMessages(messages);

The code selection are passed in as <code-selection>...</code-selection> messages. So when I try and pass in a prompt as a code selection (pinned file), Navie doesn't treat the pinned file as a custom prompt; it's treated as a "code selection".

Example prompt:

Create types to represent all the code that you find in the search context.

Start by tracing how pinned items reach Navie and how openai-completion-service handles the code-selection messages shown in the issue. Confirm that a pinned custom prompt is transmitted and interpreted as a prompt rather than as code selection, then verify the example prompt behaves accordingly.