Remove version markers from docs and source code

[vidit-admin]

Shipped-version and beta-stage markers leak into reference docs and code, against the AGENTS.md doc-writing rule: docs/engineering.md ("Shipped in v0.0.2", "shipped v0.1.0") and app/requests/page.tsx ("is a v1.0 item"). The docs-pairing gate doesn't catch this (it only checks that docs/ and planning/ are both touched); a lightweight grep gate would.

Imported from planning/backlog.md.

Note: the leftover frontend/src/app/requests/page.tsx milestone comment is fixed in PR #372.

[vidit-admin]

Duplicate of #502: #502 bundles the version-marker cleanup with the other doc passes.