[FEATURE] Configurable naming convention for auto-generated session names

When running several concurrent sessions (interactive or background jobs), the auto-generated names, like the default <dir>-<2char> or the AI-summarized titles, don't reliably distinguish sessions at a glance. /rename and claude -n <name> work, but they're manual per session, so the problem resurfaces every time a new session starts.

Request: A way to set a naming rule once (e.g., a settings.json field or a session-start hook) that shapes how the auto-generated name is produced. For example, I usually rename sessions to kebab-case because it's faster to scan and my eyes are used to it. Right now that means manually overriding the fixed default or AI-generated title every time, instead of setting the convention once.

This matters most for people who have multiple background jobs at once, where scanning a job list full of similar-looking auto-generated names isn't muscle memory.