Add ability to regenerate session title using small model #11988

Summary

Currently, session titles are only generated automatically on the first user message. There's no way to:

1. Regenerate a title if the auto-generated one is poor
2. Generate a title for sessions that were created before the title feature existed
3. Use the small model to suggest a better title mid-conversation

Proposed Solution

Add a "Generate title" option that uses the small_model (or falls back to the title agent's configured model) to create a new title based on the conversation history.

Features

1. API Endpoint: POST /session/:id/generate-title

   • Optionally accepts a custom prompt/hint as a body parameter
   • Returns the generated title

2. UI Integration:

   • Add "Generate title" item to the session dropdown menu (between Rename and Archive)
   • Show in both webapp and TUI

3. Slash Command: /retitle [hint]

   • No argument: generates title from conversation
   • With argument: uses the hint to influence title generation (e.g., /retitle focus on the auth refactoring)

Implementation Notes

• Reuse the existing title agent and PROMPT_TITLE
• Extract title generation logic from ensureTitle() in prompt.ts into a reusable function
• Remove the guards that prevent regeneration (isFirst, isDefaultTitle checks)

Use Cases

• User wants a more descriptive title after conversation has evolved
• Auto-generated title was too generic or missed the main topic
• User imported/continued an old session that never got a title