[Feature] Start session without sending an initial prompt

Problem

Currently, when launching the Claude Code CLI, a session is not actually initialized until the user sends a first message (a question, a command, or any token). This means that even a trivial "hi" / "ping" must be sent just to "warm up" the session, which:

• consumes tokens unnecessarily,
• adds a meaningless turn to the conversation history,
• feels wasteful when the user only wants the session ready for later input (e.g. opening the CLI in advance, scripting, automation).

Proposed solution

Add one (or both) of the following options:

1. Auto-start session on CLI launch — an opt-in setting (e.g. "autoStartSession": true in settings.json, or a CLI flag --auto-start) that initializes the session immediately when claude is opened, without requiring an initial user message.

1. /start slash command — an explicit, user-triggered command that initializes the session on demand, with no prompt content sent to the model.

Benefits

• No wasted tokens on placeholder messages.
• Cleaner conversation history (no dummy first turn).
• Better ergonomics for users who launch the CLI ahead of time or in automated workflows.

Environment

• Claude Code CLI
• Any platform (Windows / macOS / Linux)