Decide & schedule the domain sunset window

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Problem: Running two live domains indefinitely fragments links and search indexing with no end date.

Area: Infra / Product decision

Proposal: Set an explicit grace-period length before the old domain starts redirecting to the new one, then implement that redirect as a Worker routing rule once the window closes.

Deliverables:

• A documented sunset date.
• A redirect rule implemented once that date arrives, preserving path and query string.

Acceptance criteria:

• The old domain keeps working, unredirected, for the entire agreed window.
• Once live, the redirect preserves the full path and query string for every route, including the docs site and API subdomain equivalents.

Test scenarios:

• Verify redirect behavior for the root path, a docs page, and an API-equivalent path.

Resources:

• Worker route configuration.

Boundaries:

• Do not redirect before the agreed date under any circumstances — self-hosters and external links depend on the old domain continuing to work during the window.

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Part of #4761.

[linear-code]

JSO-3437

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Tracked follow-up from #5339: `VITE_GITTENSORY_API_ORIGIN` is a build-time hostname variable (currently `https://gittensory-api.aethereal.dev\`) that Vite bakes directly into the gittensory-ui bundle at build time. It's passed at four workflow call sites: `.github/workflows/ci.yml` (lines 170, 417), `ui-deploy.yml` (line 39), `ui-preview.yml` (line 50), and `ui-preview-deploy.yml` (line 146).

It lives entirely outside `wrangler.jsonc`, so it's easy to overlook when this issue's domain-sunset window is scheduled. Whatever the var's eventual new name/value is, it needs to land across all four call sites in the same change — noting it here so the domain cutover work doesn't silently miss it.