Add session cleanup/delete commands

Feature Request

Add built-in commands to list, archive, and delete old conversation sessions.

Current Behavior

Sessions accumulate over time with no way to clean them up from the CLI. The only options are:

• Manually deleting .jsonl files from ~/.claude/projects/
• Using the desktop app's archive feature (not available in CLI)

Proposed Solution

Add session management commands, for example:

• /sessions list — list all sessions with dates, sizes, and message counts
• /sessions delete <id|name> — delete a specific session
• /sessions prune --older-than 30d — bulk delete sessions older than N days
• /sessions archive <id|name> — archive without deleting

Alternatively, add delete functionality to the existing /resume picker (e.g., press D to delete the highlighted session).

Motivation

Power users accumulate many sessions per project. Without cleanup tools, the session picker becomes cluttered and disk usage grows. A quick prune command would help keep things tidy.