Agents were reusing <ycode-inline-variable> in custom code head,
which is not interpolated and breaks per-item JSON-LD. Document
{{FieldName}} for custom code vs inline variables for SEO.

MCP-authored [text](url) links were stored with a non-canonical mark
shape ({ href, linkType }) that the renderer's LinkSettings resolver
ignored, so the href was dropped on published/previewed pages.

- Emit canonical url variable attrs from the MCP inline parser
- Tolerate the legacy { href, linkType } shape in getLinkSettingsFromMark
  so already-authored links resolve at render time
- Normalise attrs via getLinkSettingsFromMark on the React render path
- Fix the sample template link to use the canonical shape