Spec: optional in-feed $schema for editor autocomplete

[itsbdell]

Problem

JSON Schema convention is for instances to declare `"$schema": ""` so
editors (VSCode, JetBrains) can offer autocomplete and inline
validation against the linked schema. Right now an `apps.json` is
just JSON and authors get no editor help.

Acceptance criteria

• The spec recommends (does not require) authors include
  `$schema: "https://apps-json.org/schemas/v1.json\"\` at the top
  level of their feed.
• The schema declaration tolerates the field (allows it as an unknown
  property; we already do because `additionalProperties` is true at
  the root).
• The example feed includes it, demonstrating the pattern.
• Spec language: an absent or differing `$schema` is not an error
  — the validator decides which schema to use, the field is purely
  hint-for-tooling.

Notes

Aligns with how editors handle `package.json`,
`tsconfig.json`, etc. Zero validation cost; pure UX win for authors.