#25121 cmux NEXT sidebar: group restored Cloud machines

HOLD: structural sidebar change; awaiting Leo dogfood build.

Cloud machines restored before the remote-group flow stayed headerless in the one-list sidebar. Their host_group flag was only marked by the new-workspace creation action, so opening an existing machine or starting after an upgrade did not show one collapsible Cloud row with its sessions underneath.

This adds a sidebar-document lifecycle pass: after the document-backed sidebar becomes active, every currently named Cloud session is marked once as a host group. The reducer-plan regression covers an existing machine whose flag is false; the existing Cloud/nested-group layout tests remain in the focused suite.

Validation at exact head 45cce3cfe3216a39d3ab640ba3f9b750a12ed727:

• python3 scripts/cmux-next/prepush-gate.py — passed.
• python3 scripts/verify-local.py — passed (4/4 selected static checks; native compile/app launch deferred to fleet).
• cmux-ci step 4e11fe4128ef6b0cde3373de on baseline 56206f8a2e4369352d72079c3dcda758df52ed2d: SidebarCloudRemoteGroupTests and SidebarHostGroupLayoutTests passed (the repair head was not yet pushed when this receipt ran).

Capture evidence: before, SidebarBridge.show had no lifecycle mark and restored sections remained host_group=false; after, markCloudMachineGroups() runs immediately after SidebarDocMachines.drag() becomes active, with SidebarDocHostGroupPlan setting host_group=true. Design reference: plans/cmux-next/remote-groups-mocks/index.html recommended host-header + nested-group layout.

Agent session: big-red

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