Allow renaming sessions shown in /resume list

Feature Request

The /resume command shows a list of recent sessions with auto-generated titles derived from the first message. There is currently no way to rename or label a session.

Use case

When working on a long-running task across multiple days (e.g. "docker build server infrastructure"), the auto-generated title may not be descriptive enough to identify the right session in the /resume list. Users need a way to mark or rename sessions with a meaningful label.

Proposed solution

One or more of:

• A /rename <title> command to label the current session
• An inline rename option in the /resume session picker
• A metadata field in the session file that the resume list reads as the display title