VS Code extension: support renaming/editing session names

Feature request

Environment: VS Code extension

What's missing:
The VS Code extension has no way to rename or edit session names. Sessions are auto-titled and there is no UI (or slash command) to change the name after the fact. Typing /rename in the extension returns: _"/rename isn't available in this environment."_

Expected behavior:
Users should be able to rename a session — either inline (e.g. a slash command like /rename <name>) or via the session history picker UI — so conversations can be meaningfully labeled and retrieved later.

Why it matters:
When switching between many ongoing projects and sessions, auto-generated titles make it hard to identify and resume the right conversation.

Suggested approach (optional):

• Add a /rename <name> slash command that works in the VS Code extension context
• And/or add a rename affordance (e.g. right-click or pencil icon) in the session history sidebar/picker