Feature request: `/id` command to display current session ID

Problem: No direct way to get the current session ID from within a conversation. Users must parse ~/.claude/history.jsonl or find files in ~/.claude/projects/.

Use cases:

• Save session ID in project notes for agent handoff (e.g., resuming investigation later with --resume)
• Scripts that clone/fork conversations

Proposed solution: Add /id (or /session) command that outputs the current session ID.