Add "Ask" / chat-only mode to Copilot ACP agent for Zed and other ACP clients

### Describe the feature or problem you'd like to solve

_No response_

### Proposed solution

## Summary
When using GitHub Copilot as an ACP (Agent Client Protocol) external agent in Zed IDE,
the only available modes are **Agent**, **Plan**, and **Autopilot**. There is no
lightweight "Ask" or chat-only mode for free-form Q&A without agentic behavior.

## Expected Behavior
An "Ask" mode (or equivalent) should be available via ACP that allows users to:
- Ask questions about their codebase
- Have general conversations with the model
- Get answers and explanations WITHOUT the agent autonomously making file edits or
running commands

## Current Behavior
Only three modes are exposed to ACP clients:
- Agent
- Plan
- Autopilot

All three are agentic in nature. There is no pure conversational/read-only mode.

## Why This Matters
Zed's own built-in agent profiles include "Ask" (read-only tools) and "Minimal"
(no tools, pure chat) — but these profiles **only apply to Zed's first-party agent**.
When Copilot runs as an external ACP agent, it overrides Zed's profile system entirely
and only surfaces its own modes.

Users who want to simply chat with Copilot models (e.g. ask a question, get an
explanation, discuss architecture) without triggering agentic workflows have no way
to do so inside Zed when using Copilot via ACP.

## Workaround
Currently none within Zed + Copilot ACP. Users must either:
- Switch to a different provider (Anthropic, OpenAI, etc.) to get the Ask/Minimal profile
- Use github.com/copilot in the browser for free-form chat

## Environment
- Zed IDE: v1.0.0 (April 29, 2026)
- Integration: GitHub Copilot via ACP (external agent)
- Affected ACP clients: Zed (confirmed), likely any other ACP-compatible client

## Request
Please add an **"Ask"** mode (or similar read-only/chat-only mode) to the Copilot
ACP agent implementation so that ACP clients like Zed can expose it as a profile option.

Related discussion: zed-industries/zed#48346

### Example prompts or workflows

_No response_

### Additional context

_No response_

The issue names no files or tests. Start at the Copilot ACP agent implementation and trace how Agent, Plan, and Autopilot modes are exposed to ACP clients. Define and expose an Ask-equivalent read-only/chat-only mode, then verify an ACP client such as Zed can select it without file edits or command execution.

Scritto dal modello di indicizzazione a partire dal testo della issue.