Add /rename slash command support in VSCode extension

Feature request

The /rename command for renaming conversation sessions works in the CLI (claude --resume → press R), but is not available in the VSCode extension.

Typing /rename in the VSCode input box shows "No matching commands".

Expected behavior

/rename <name> should work in the VSCode extension input box, the same way other slash commands do. This would let users rename the current conversation without leaving the IDE.

Why it matters

When working on multiple tasks, conversations pile up with auto-generated names. Being able to rename them from within the extension makes it easier to find and resume past sessions.