xtctx: v0.22.1

xtctx no longer points at any hosted sync service. Cloud sync stays available
for people who deploy their own Worker.

Changed

• Cloud sync is self-hosted only. There is no default sync server any
  more. xtctx login without --sync-url or XTCTX_SYNC_URL exits 1 with a
  message pointing at docs/cloud-sync.md and cloud/README.md, and contacts
  nothing. XTCTX_TOKEN with no URL fails the same way. (#411)
• The Worker config in the repository ships with placeholders. A deployment's
  own values go in a git-ignored cloud/wrangler.local.toml, copied from
  wrangler.local.example.toml; until PUBLIC_URL is set the Worker answers
  500 server_misconfigured on every route except /health.

Removed

• The hosted Worker that served sync.xtctx.com and mcp.xtctx.com during
  0.22.0, and every default pointing at those addresses.