Add ability to rename sessions for --resume

Problem

When resuming sessions with claude --resume, sessions are identified by their first user message. If that message is a URL (e.g. a Slack link) or otherwise non-descriptive, there's no way to identify the session later.

There's no built-in command to rename/retitle a session.

Expected behavior

A command like /title <name> or /rename <name> that sets a human-readable title for the current session, displayed in the --resume picker.

Current workaround

None — you just have to remember which cryptic first message corresponds to which task.

Context

Common workflow: paste a Slack/GitHub URL as the first message, then do extensive work. The session becomes impossible to find later in --resume without trial and error.