[Feature Request] Desktop Code tab: invoke custom subagents via @-mention or agent picker

Problem
In the CLI, custom subagents defined in ~/.claude/agents/*.md can be invoked explicitly with @agent-<name>, or a whole session can be started as an agent with claude --agent <name>.
In the Claude desktop app's Code tab, neither is available: typing @<agent-name> only suggests sessions and files (e.g. it matched an unrelated session with a similar title), and there's no UI to start a session as a given agent. The only option is asking in plain text ("use the X agent…") and hoping the main model delegates.

Request

1. Show custom subagents in the @ autocomplete in the Code tab (like @agent-<name> in the CLI), so the user can pick one explicitly.
2. Add an option when starting a new session to run it as a specific agent (the GUI version of claude --agent <name>), or clearly document whether the "agent" setting in settings.json is honoured by the desktop app.

Why
We keep domain-specific agents (e.g. one per customer project) with detailed instructions. Being able to invoke them explicitly and reliably from the GUI, with interactive follow-ups, would make them much more useful for non-CLI users.

Environment
Claude desktop app (Code tab), Windows 11.