Edit in sandbox (create + picker)

[elias-ba]

As a workflow builder on a live workflow, I want to branch it into a sandbox and edit there instead of touching main, so that I can make and test changes safely while production keeps running on the current live version.

Part of #4852 (Sandbox DevX epic).

Outcome

From a live workflow, Edit in sandbox opens a picker to create a new sandbox or join an active one, and lands you in the sandbox editor with the workflow's trigger live.

Scope

• Edit in sandbox picker modal (new component): "Create new sandbox (branch from the current live version)" plus a list of active sandboxes to join, sorted by last-edited time, each showing the last editor and collaborators.
• "Create new" goes through Lightning.Projects.Sandboxes.provision/3 (full-clone, no workflow subsetting at creation). "Join" opens the workflow in the sandbox editor. New WorkflowChannel events.
• On opening a workflow for editing in a sandbox, enable that workflow's trigger in the sandbox (deliberate divergence from clone_triggers_with_disabled_state).
• sandbox badge in the editor header.

Acceptance criteria

• Given a live workflow, when I click Edit in sandbox, then a picker opens with Create new and a list of joinable active sandboxes.
• Given Create new, when it completes, then a full-clone sandbox is provisioned, I land in the sandbox editor, the workflow is editable, and a sandbox badge shows.
• The workflow's trigger has its own /i/... endpoint and is live in the sandbox, and main is unchanged.
• When I reopen the picker, the active sandbox is listed to join, sorted by last-edited.

Key surfaces

New picker component, Sandboxes.provision/3, WorkflowChannel, editor header, awareness/presence (useAwareness, ActiveCollaborators).

Dependencies

Builds on the lifecycle foundation. Phase M2 (sandbox round trip). In review as PR #4901.

Estimate

8 story points.