Plugin-sourced slash commands in a remote/cloud Code session report "Unknown command" instead of explaining plugins aren't available there

Plugins installed locally (via CLI or Desktop's plugin browser) aren't available in remote/cloud Code sessions — this is documented behavior (desktop.md: "The plugin browser is not available in cloud sessions, and plugins you install from the desktop app aren't available for cloud sessions").

However, when a user invokes a plugin-sourced slash command (e.g. /my-plugin:my-command) while a remote/cloud environment is selected in Desktop's Code tab, the response is a generic Unknown command: /my-plugin:my-command — identical to the error for a command that simply doesn't exist. There's no indication that the command does exist and would work, just not in this session type.

Reproduction: install any plugin locally, confirm it works in a local Code session, then switch to a remote/cloud environment in the same Desktop Code tab and invoke the same slash command.

Expected: an error that distinguishes "this command doesn't exist" from "this command exists but isn't available in remote/cloud sessions — switch to a local session to use it," ideally suggesting the fix.

Actual: identical generic "Unknown command" for both cases, leading users to suspect their plugin installation rather than their session type.