Tag-filtered CV variants (/cv/backend/)

From LATER.md: tag-filtered CV variants (`/cv/backend/`, `/cv/elixir/`) for targeted applications. The portfolio tag machinery (story pages, skills) already computes everything needed; this is a routing + curation feature. Unlisted-by-default probably right (shareable URL, out of sitemap/llms.txt), like the existing CV visibility model.

Start with LATER.md, then trace the existing portfolio tag machinery for story pages and skills and the current CV visibility model. Define the route generation and curation behavior for tagged CV variants, ensuring the resulting URLs are shareable but excluded from the sitemap and llms.txt.

Written by the indexing model from the issue text.