[FEATURE]: Rich grouped Add (+) menu in chat input

Feature hasn't been suggested before.

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Describe the enhancement you want to request

The chat input + ("Add") menu is too simple today: a flat list (attach files, / commands, @ context, ! shell). Make it a rich grouped menu like modern assistants:

• Images and files — attach (shortcut Ctrl+U).
• Commands — section listing slash commands with trigger + description inline (not just an entry that opens another picker).
• Context — entry points (files and folders, agents, MCP resources) with descriptions.
• Shell command — switch to shell mode, with description.
• Plugins — installed plugins/MCP servers with icon + name + description, when available to the client without new endpoints.

Each item should show icon + name + description + keyboard hint, reusing the existing popover item pattern. Searching GitHub issues found no existing request for this grouped Add menu.