feat(frontmatter): support nested keys via dot-notation

[jackchuka]

Summary

Resolves #59. Adds support for validating nested YAML frontmatter keys using dot-notation field names.

• New object field type for asserting a parent is a map.
• name: "metadata.author" traverses nested maps; works with both map[string]any and map[any]any (yaml.v3 may emit either for nested maps).
• Backslash-escape (weird\.key) supports literal dots in segment names.
• Generator emits properly indented nested YAML when multiple paths share a parent.

Example

frontmatter:
  fields:
    - { name: "title" }
    - { name: "metadata", type: object }
    - { name: "metadata.author" }
    - { name: "metadata.version" }
    - { name: "metadata.homepage", optional: true, format: url }

Validates documents like:

---
title: My Document
metadata:
  author: example-org
  version: "1.0"
---

Test plan

• go test ./... — all green
• go run ./cmd/mdschema check on a valid doc → no violations
• Same on a doc missing metadata.author → reports Required frontmatter field 'metadata.author' is missing
• go run ./cmd/mdschema generate produces nested YAML with proper indentation
• schema.json regenerated and committed