[Feature Request] Support command arguments syntax for `/plan` (e.g., `/plan description`)

Bug Description
I often want to tell claude code to plan something and I will instinctively type something like /plan blah blah blah where "blah blah blah" is the description of what I want to plan. However, this just enables plan mode and it ignores the plan prompt.

I will then come back a few minutes (or more!) later and find that all that has happened is that plan mode is now enabled, but it hasn't actually started the plan.

The muscle memory of the convention of a "command" being followed by "arguments" is just so strong (along with the fact that usually whatever you type is given as instructions to the agent) that I keep making this mistake.

It's likely that this same feedback could apply to all the /commands

Environment Info

• Platform: darwin
• Terminal: Apple_Terminal
• Version: 2.1.63
• Feedback ID: 7d4bb207-2b8d-4689-b8af-ba9d26e7ee08