Allow concise input declarations for Markdown components

[taras]

Why

Markdown components currently repeat the enclosing object schema even when they
only need to declare a few named inputs:

inputs:
  type: object
  properties:
    name:
      type: string
  required: [name]
  additionalProperties: false

The object shape is an implementation invariant, not useful information for
most document authors. A concise form would make input declarations easier to
read while preserving the existing JSON Schema contract internally.

Proposed authoring form

required: [name]

inputs:
  name:
    type: string
  loud:
    type: boolean
    default: false

Normalize this before compilation to:

type: object
properties:
  name:
    type: string
  loud:
    type: boolean
    default: false
required: [name]
additionalProperties: false

Property definitions remain draft-07 JSON Schema. The shorthand only makes the
enclosing closed object implicit.

Contract

• Shorthand applies to every Markdown component, whether imported or executed
  as the root document.
• inputs.type: object selects the existing full JSON Schema form.
• Otherwise inputs is a map of property names to property schemas.
• Top-level required belongs to shorthand.
• Full JSON Schema keeps required inside inputs.
• Supplying top-level required with the full form is a configuration error;
  the two lists are never merged.
• Shorthand normalizes to additionalProperties: false.
• An absent inputs declaration retains the existing empty closed schema.
• Frontmatter inspection and execution use the same normalization path.

Acceptance

• A shorthand component receives and validates props exactly like the
  equivalent full schema.
• Required properties and recursive defaults behave identically in both forms.
• Imported and root Markdown components share the behavior.
• Invalid property schemas fail during frontmatter/schema validation.
• Ambiguous or mixed shorthand/full declarations fail with a useful
  diagnostic.
• Existing full schemas remain unchanged.

Scope

This issue does not add positional command-line arguments. That work consumes
the normalized schema in a separate follow-up.

Related: #171