ci:prepare fails: _get:docsy derives a Go module query from the npm version field

[chalin]

_get:docsy runs hugo mod get github.com/docsy/docsy/theme@v$npm_package_version, but the version field carries build metadata (0.17.1-dev+NNN-over-main-HASH), which is not a Go module revision. Go rejects it (invalid version: unknown revision theme/v0.17.1-dev+...), so npm run ci:prepare and ci:test exit 1.

• Pre-existing (the same shape fails on main before Upgrade Docsy to the 0.18.0 pre-release and Hugo 0.166.0 #519); not on a live path: .github/workflows/links.yaml never calls ci:prepare, and update:docsy:* don't interpolate the version.
• Fix direction: have ci:prepare trust the pin already in go.mod (hugo mod graph && hugo mod tidy, dropping _get:docsy), or derive the @ query from go.mod rather than from the npm version.

Found during the review of #519.