Allow manual session renaming for easier identification in /resume

Problem

When resuming sessions with /resume, it's very hard to identify which session is which because session titles are auto-generated from conversation content.

The auto-generated titles are often unhelpful — they pick up fragments of conversation that don't represent what the session was actually about. There's no way to manually set or rename a session title.

Proposed Solution

Allow users to manually rename/set session titles. For example:

• A /session rename "my title" command
• Or a --title flag when starting a session
• Or an interactive rename option in the /resume list

Why This Matters

• When working on multiple projects or long-running tasks, sessions pile up quickly
• The auto-generated titles make it nearly impossible to find the right session to resume
• This is especially painful for users who frequently switch between sessions or machines

Environment

• Claude Code CLI (Windows / macOS)