Add ability to delete/clean up sessions from /resume list

The /resume picker accumulates sessions over time, including trivial ones with only a few messages. There's currently no way to delete or clean them up.

Suggested features

• Add a D (delete) keybinding in the /resume picker to remove individual sessions
• Option to bulk-delete sessions by age or message count (e.g., remove sessions with <3 messages)
• A /sessions cleanup command for batch maintenance

Current workaround

Manually deleting files from ~/.claude/projects/*/sessions/, which is undocumented and fragile.

Use case

Long-term users on active projects accumulate dozens of short throwaway sessions that make finding meaningful ones harder.