Auto-rename sessions after first exchange

Problem

Session names are most useful when they reflect the actual conversation topic, but that's unknowable at session start. Today, /rename (no args) auto-generates a good name from conversation history — but you have to remember to invoke it manually every time.

This is a small friction that adds up. Most sessions never get renamed, so /resume lists a wall of unnamed sessions.

Proposal

Add a --auto-rename flag (or equivalent setting) that automatically names the session after the first assistant response, using the same logic /rename already uses.

claude --auto-rename

Or as a persistent setting so it's always on:

{
  "autoRename": true
}

The timing (after the first exchange) is ideal because there's enough context to generate a meaningful name, but it's early enough that the name is visible for the rest of the session.

Alternative

Expose session renaming as a CLI subcommand (e.g., claude session rename <id>) so that hooks can call it. A Stop hook could then handle this. But this is more machinery for the user to wire up — --auto-rename / a setting is the better UX since it's zero-config after enabling.