Allow customizing auto-generated session/conversation titles

Problem

Session titles in VS Code extension are auto-generated from the initial prompt, but there's no way to control:

1. Language — titles default to English even when the user's working language is Russian (or any other non-English language)
2. Content pattern — no way to specify what part of the prompt to use (e.g., first N words, a regex pattern)

Current behavior

• Titles are auto-generated in English regardless of the conversation language
• The only workarounds are manual: /rename or claude -n "name" at startup
• Inconsistent: sometimes the auto-generated title respects the prompt language, sometimes it doesn't

Desired behavior

A setting (in settings.json or CLAUDE.md) to control session title generation, for example:

{
  "sessionTitle": {
    "language": "ru",
    "source": "first_words",
    "maxLength": 50
  }
}

Or at minimum, respect the language of the user's first message when generating the title.

Use case

I work entirely in Russian and name my tasks with work product numbers (e.g., "WP-130 архитектура Phase 2"). Every session I have to manually /rename because the auto-generated title is in English and doesn't reflect my naming convention.

Alternatives considered

• /rename — works but requires manual action every session
• claude -n — only available from CLI, not from VS Code extension
• CLAUDE.md instructions — tried adding "use Russian for session titles" but it has no effect on the auto-naming logic