/rename session names not persisted to disk

Description

/rename accepts a session name and confirms it (Session renamed to: game-design), but the name is not persisted to disk. After exiting and resuming, the name is lost. claude --resume game-design cannot find the session.

Steps to Reproduce

1. Start a Claude Code session: claude
2. Rename it: /rename game-design
3. Observe confirmation: Session renamed to: game-design
4. Exit: /exit
5. Try to resume: claude --resume game-design — fails to find the session

Root Cause

sessions-index.json has no name or title field in its schema. The current fields per entry are:

{
  "sessionId": "uuid",
  "fullPath": "...",
  "fileMtime": 1234,
  "firstPrompt": "truncated first message",
  "messageCount": 71,
  "created": "...",
  "modified": "...",
  "gitBranch": "main",
  "projectPath": "...",
  "isSidechain": false
}

The auto-generated slug (e.g. "fluttering-floating-origami") is stored inside the JSONL transcript but is not the user-provided rename value.

Expected Behavior

/rename game-design should persist the name to sessions-index.json (e.g. as a name field), and claude --resume game-design should resolve it.

Environment

• Claude Code v2.1.34
• Linux (GCP VM, Ubuntu)
• Running 3 parallel sessions in the same project directory