Add chat:rename action for keybinding support

Feature Request

Problem

/rename is a useful slash command for renaming the current session, but it cannot be bound to a keyboard shortcut. The keybinding system only supports built-in actions (e.g., chat:submit, chat:newConversation), and there is no chat:rename action available.

For users who frequently rename sessions (e.g., after the topic becomes clear in the first few turns), typing /rename every time adds unnecessary friction.

Proposed Solution

Add a chat:rename action to the keybinding system that triggers the rename prompt for the current session, equivalent to typing /rename.

This would allow users to bind it in keybindings.json:

{
  "context": "Chat",
  "bindings": {
    "ctrl+n": "chat:rename"
  }
}

Use Case

• Power users who manage many concurrent sessions and rename them early in the conversation
• Consistent with the existing pattern where other slash commands have keybinding equivalents (e.g., model picker, thinking toggle, diff view)

Additional Context

• #23363 previously requested the /rename command itself (closed as duplicate since it was implemented)
• This request is specifically about exposing it as a bindable keybinding action