[Feature Request] Support keybindings for user-defined slash commands

Bug Description
Title: Allow keybindings to invoke user-defined slash commands

Body:

Currently ~/.claude/keybindings.json bindings can only trigger actions from a fixed built-in namespace (chat:submit, app:toggleTodos, etc.). There's no way to bind a key to a user-defined slash command in ~/.claude/commands/<name>.md.

This is a meaningful gap because slash commands are the extension surface users build personalized workflows on top of, but power-user UX (one-key invocation) is closed to them.

Concrete use case: I built a /guide slash command that pauses the current task and presents an AskUserQuestion menu (Continue / Redirect / Show status / Fork / Abort). It's invaluable for redirecting mid-session. I wanted to bind Alt+G to it so I don't have to type the whole command, but no action type supports that.

Proposed shapes (any one would solve it):

// Option A — explicit slash action
{ "key": "alt+g", "action": "slash:guide" }

// Option B — generic text-typing action
{ "key": "alt+g", "action": "typeText", "text": "/guide\n" }

// Option C — flat string, "slash command if starts with /"
{ "key": "alt+g", "command": "/guide" }

Bonus value: this would also unlock keybindings for plugin-namesperpowers:brainstorming, which are even more typing to invokemanually.

Currently I'm working around it by displaying a > Lost? type /guide hint in my statusline so users at least see the option. A real binding would be much better.

Environment Info

• Platform: win32
• Terminal: windows-terminal
• Version: 2.1.119
• Feedback ID: a9a3e02d-ebcb-468e-bcfd-e9419e151bde