Move the popover's prepare-summary reducer (and the
`isFallbackNoticeStep` helper duplicated word-for-word in
`prepare-progress.tsx`) into `apps/web/lib/prepare/summarize.ts`. The
hook moves to `hooks/domains/session/use-prepare-summary.ts` to match
the project's domain-organized hook layout.
The popover's PrepareSummary type and the chat panel's EffectiveStatus
serve different UIs, so the panel keeps its own `deriveStatus` — only
the literal duplicate (`isFallbackNoticeStep`) is shared.
When the only warning on a completed prepare is the missing-sandbox
fallback notice, render "Environment prepared on a fresh sandbox" in
the header instead of the generic "Environment prepared with warnings".
The fallback row still carries its own amber warning callout so the
user understands why a fresh sandbox was provisioned.
José Almeida