[FEATURE]: command config: add ability to disable/hide individual MCP-sourced commands

Verification

• I have searched the existing issues and this has not been reported before.

Problem

MCP servers that expose prompts (via prompts/list) get automatically registered as slash commands in the TUI (source: "mcp"). There is currently no way to disable or hide a specific MCP prompt from appearing as a command, without disabling the entire MCP server.

This is relevant when an MCP server exposes a prompt that is irrelevant or undesirable for a particular project (e.g., dart-mcp-server:flutter_driver_user_journey_test from the Dart MCP server — a user-journey-to-test pipeline that not every Flutter project needs), but the other tools from that same server are actively used.

Current behavior

• command config only supports adding custom commands, not disabling existing ones.
• tools config with false only disables MCP tools, not MCP prompts.
• No permission-based mechanism to suppress MCP commands either.
• agent config supports disable: true to suppress built-in agents, but commands have no equivalent.

Desired behavior

Allow disabling specific commands (including MCP-sourced ones) via config, e.g.:

{
  "command": {
    "dart-mcp-server:flutter_driver_user_journey_test": {
      "disable": true
    }
  }
}

Use case

The Dart MCP server (dart mcp-server) ships with the Flutter SDK and exposes 13 tools + 1 prompt. The prompt (flutter_driver_user_journey_test) is a full user-journey-to-test pipeline that many projects don't need, but disabling the entire MCP server would lose access to useful tools like widget_inspector, hot_reload, flutter_driver_command, analyze_files, etc.

Environment

• opencode 1.18.31
• macOS / any platform