Add support for slash commands in `copilot --acp`

[cocoich]

Describe the bug

Copilot ACP doesn't provide available_commands_update, so ACP frontend is unable to use slash commands.

Affected version

0.0.388

Steps to reproduce the behavior

(in zed)

1. add Custom agent like

  "agent_servers": {
    // ...
    "Copilot Cli": {
      "type": "custom",
      "command": "copilot",
      "args": ["--acp"],
    }
  },

2. try /model
3. error with Available commands: none

Expected behavior

No response

Additional context

• Zed 0.219.5
• zed impl: https://github.com/zed-industries/claude-code-acp/blob/36ea238d76490c60a68150e926095ef5d32ea759/src/acp-agent.ts#L796-L805
• acp docs: https://agentclientprotocol.com/protocol/slash-commands