Feature request: user-configurable top-bar buttons to invoke custom commands/agents

Feature request

Let users add custom buttons/icons to the Claude Code top bar (desktop/IDE UI) that invoke a configured slash command or agent.

Problem

Custom agents and commands (defined in .claude/commands/, .claude/agents/, or plugins) have no visual presence in the UI. The top bar is not extensible, so a custom agent someone uses all day (e.g. a personal review agent invoked as /sage) is invisible — discoverable only by remembering to type it. For non-expert users of a shared setup, if it isn't clickable it effectively doesn't exist.

Proposed solution

A small declarative surface, e.g. in settings or the command/agent frontmatter:

{
  "toolbar": [
    { "icon": "🧠", "tooltip": "Talk to Sage", "command": "sage" }
  ]
}

Rendered as a clickable icon in the top bar; clicking inserts/runs the slash command in the current session. Emoji-as-icon would be enough for v1.

Why it matters

It makes custom agents first-class: today everything shipped in the product has UI affordances, while everything user-built is typing-only. Together with keybinding support for custom commands (filed separately), this would close the gap between built-in and user-defined workflows.