[FEATURE] /btw with no arguments should open the side chat in the CLI (parity with Desktop), and add a composer link for it in Desktop

Current behavior

• Claude Code CLI: Typing /btw with no argument only prints a usage message:

``
❯ /btw
⎿ Usage: /btw <your question>
``

• Claude Desktop (Code tab): Typing /btw with no argument opens the side chat, where you can have a back-and-forth conversation.

Requested

1. CLI parity: When /btw is run with no argument in the CLI, open an interactive side chat, like Desktop does, instead of showing the usage message. /btw <question> can keep its current one-shot behavior.
2. Desktop discoverability: Add a link/button for the side chat under the composer, next to the existing Artifacts, Shell and Monitor links, so it can be opened with one click. Right now you can only open it by typing /btw or with a keyboard shortcut that's hard to find.

Why

Right now the same command does different things in the two apps, which is confusing. The side chat is very useful for quick questions that shouldn't interrupt the main task, but it's hard to discover without a visible entry point.

Related

• #93019 (configurable panel buttons in the Desktop toolbar)