Allow /fork during an active run instead of forcing wait or cancel

Summary

/fork should remain usable while Claude Code is actively running, instead of being blocked until the current run finishes.

Current pain

When Claude Code is in the middle of an active run, I sometimes realize I want to branch immediately.
Right now that means I have to either:

• wait for the run to finish, or
• cancel the run just to fork

That is exactly the wrong moment to remove branching control.

Requested behavior

Make /fork a control-plane command that can be handled during an active run.

Reasonable behaviors could include:

• immediate fork from the current conversation state
• or queue /fork as a high-priority control action without forcing a cancel
• or interrupt safely, fork, and preserve the current run state in a predictable way

The important part is that it should not be a hard "you can't do that right now" block.

Why this matters

Forking is often most useful mid-run, when I realize I want to explore another path without discarding the current direction or waiting for a long operation to end.

Blocking /fork during active execution makes the workflow more rigid right when users need the most flexibility.

Scope

This request is specifically about /fork, but the broader product rule may be:

• session-control slash commands should still work while a run is active whenever they can be reconciled safely

Additional context

I have the same gripe in Codex as well: mid-run control commands should be steerable rather than blocked.