Feature request: /fork should support opening in a new terminal tab

Summary

The built-in /fork command creates a fork of the current conversation, but it stays in the same terminal. It should support opening the forked session in a new terminal tab.

Use case

When building a complex feature, I often need to branch into multiple thinking threads that share the same context but explore different angles:

• Try different implementation approaches in parallel — "build it with approach A in one tab, approach B in another"
• Explore tangential concerns without derailing the main thread — "how would this affect the auth system?" or "what if we need to support X down the line?"
• Separate building from reasoning — keep the implementation moving in one tab while discussing architecture and long-term implications in another

These are multiple "discussion rooms" that share the same starting context but each need their own space. The workflow should be: hit /fork, a new tab opens with full context, and I'm immediately in a parallel conversation exploring a different angle — without interrupting the original session.

Currently this requires manually opening a tab, navigating to the directory, and resuming a forked session. The friction kills the flow.

Current workaround

I built a custom /fork-tab command that:

1. Echoes a unique token (which gets written to the session's .jsonl file)
2. Greps all session .jsonl files for that token to identify the current session ID
3. Uses AppleScript to open a new terminal tab and runs claude --resume <id> --fork-session

This works but is a hack — the token-grep approach is necessary because there's no way to access the current session ID from within a session (no env var, no API, process doesn't keep the .jsonl file open).

Proposal

Two changes that would make this much cleaner:

1. Expose CLAUDE_SESSION_ID as an environment variable in the bash tool subprocess. This eliminates the need for the token-grep hack and enables session-aware tooling.

1. Add a --new-tab flag to /fork (or a separate /fork-tab command) that opens the forked session in a new terminal tab, with support for Terminal.app and iTerm2.