Keyboard shortcut to fork from the latest response

[hh36000]

Problem

Forking a thread only works by mouse: hover a reply, then click "Fork from this response". Most other thread actions already have a command you can bind (thread.copyReference, thread.settle, thread.pin, thread.undo), but forking doesn't. Even rebinding is impossible, because keybindings.json only accepts commands from the built-in list.

The common case is forking from the reply you just read, which is the latest one in the thread.

Proposal

Add a thread.fork command that forks from the latest response in the active thread that can be forked. It does exactly what clicking the button on that reply does today: create a "<title> fork" thread and open it. If no reply in the thread can be forked, the shortcut does nothing.

Default key: mod+shift+y (currently unused) with when: "!terminalFocus", or left unbound if you'd rather people opt in.

Scope

• Contracts: add "thread.fork" to THREAD_KEYBINDING_COMMANDS in packages/contracts/src/keybindings.ts, with the default rule next to the other thread.* rules.
• Web: handle it in ChatView's shortcut commands map. Find the newest reply whose run can be forked, using the same canForkProjectedAssistantItem check the button uses, then call the existing onForkFromRun from useTimelineRewind. The fork path itself doesn't change.
• Settings and docs: the command appears in the keybindings settings list, and docs/user/keybindings.md gets one row.
• Mobile: left out, since it has no keyboard shortcuts.

The change is small. I can open a PR if the direction looks fine.