[FEATURE] Auto-generate descriptive session titles for easier history browsing

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Browsing conversation history in the VS Code extension and claude --resume is frustrating. Sessions are only identified by timestamp and the raw first prompt. When you have 20+ sessions on the same project, finding the right one means clicking through each to preview content.

Proposed Solution

After each session ends, automatically call a fast/cheap model to generate a 5-10 word descriptive title (e.g. "Debugging Lua table serialization bug", "Setting up Docker compose for local dev"). Display this title in the history panel and --resume list instead of the raw first-prompt text.

Alternative Solutions

#6006 (/rename) is a good companion feature, but is locked and requires manual effort from users — most won't bother
A simple heuristic (first N words of first prompt) is better than nothing but still vague

Priority

Critical - Blocking my work

Feature Category

CLI commands and flags

Use Case Example

_No response_

Additional Context

_No response_