Problem

Frontmatter fields in the editor are all free-text. For controlled fields — tags, type, context — that means typos and quiet drift, and there's no way to see the allowed values while editing. Re-typing a known vocabulary by hand defeats the point of having one.

Proposal

Let a folder declare allowed values per field, and have the editor render a dropdown instead of a text input.

Declaration in the existing .ok/frontmatter.yml (new, optional _schema block):

_schema:
  type:
    enum: [note, person, project, concept]
  tags:
    type: list
    enum: [docs, internal, wip]

• folder-config-get returns this as frontmatter_schema.
• FrontmatterRow routes a field that has an enum to a new SelectWidget (reusing the existing Radix <Select>): single-select for scalars, multi-select for list/tags.
• No declaration → unchanged free-text. Fully backward-compatible.

Why this shape

• Reuses what's already there — the Radix Select, and the {type:'enum', values} pattern you already use for MDX component attributes.
• Folder-scoped, so each area defines its own vocabulary.
• Additive: no change to the existing frontmatter types.

Happy to implement it if the design works for you — including where you'd prefer the declaration to live (.ok/frontmatter.yml vs a separate .ok/schema.yml).