v0.6.0

hosted-only: retire the self-host story

README, ROADMAP, ARCHITECTURE §18, and the public site now describe the
hosted service only. Remove the GitHub Action (action.yml + example) —
it booted the self-hosted compose stack, which the hosted auth/credit
model no longer supports. Drop open-source references and GitHub links
from the landing page, OG image, llms.txt, JSON-LD, and pricing page.
CLI README/pyproject lose GitHub Releases links and the self-hosted
description ahead of the repo going private. Drop stale test counts and
the removed `virgil config set api_url` from the VS Code docs.