Remove the unowned domain reference

Goal

Remove every current-source reference to the unowned legacy domain. Keep the site portable so its final production domain is supplied only at deployment time.

Scope

• Remove the old collector callback URL and require it to be configured as a deployment secret or variable.
• Remove the old domain and www variant from the public traffic response.
• Replace public documentation links with repository-relative links where possible.
• Replace domain-specific SEO and deployment guidance with placeholders for the future owned domain.
• Add an automated regression check that fails if the removed domain appears in tracked source files.

Boundaries

• Do not rewrite Git history or alter already-published historical commits.
• Do not select or configure a new domain.
• Do not change the traffic API architecture beyond removing the old host values.

Validation

1. The new regression test fails before the cleanup and passes afterwards.
2. Source search finds no reference to the removed legacy domain outside generated build output and Git history.
3. Existing automated tests and npm run docs:build pass.