issue 4531 slice 48: PublishingShell copy the open site #4961

Parent: #4531

Slice goal

PublishingShell: copy the open site to a new site name. Rename is a separate slice. One confirm creates the copy; cancel does not. The new site appears in the sites list.

Out of scope

• Delete site
• Copy of a single delivery server (already shipped)
• Full publish redesign

Acceptance criteria

• Confirm creates a distinct site and does not rename the source
• Cancel and invalid/duplicate names create nothing
• HTTP 400/403/409 stay on the workspace
• Vitest plus surface-filtered Playwright
• product-docs/8.2 updated
• Update parent [8.2] Unified publishing — next React increments #4531 Agent progress when the PR opens

Leave unassigned. Do not add In Progress.

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