Refs: #523

This pull request reserves key as a well-known optional field in the per-page JSON configuration comment, so that AI agents and external tooling can refer to a specific slide by a stable, opaque identifier instead of relying on page numbers, headings, or body fragments — all of which shift or collide in practice.

Behavior:

• key is optional and opaque; deck does not interpret its content, length, or character set.
• It has no effect on rendering and is not considered for content equality (changing only the key does not trigger a slide update).
• Empty values are treated as unset.
• Uniqueness is enforced at parse time. Two pages declaring the same non-empty key cause Parse to return duplicate page key %q at pages %d and %d, so downstream consumers can rely on the identity guarantee rather than treating it as a convention.

Example:

<!-- {"key": "a7b5", "layout": "title-and-body"} -->

# Slide title

---

<!-- {"key": "c9d2"} -->

# Another slide

Changes:

• Add Key field to md.Config and md.Content, and copy it through when parsing the page-config HTML comment.
• Add validateKeys and call it from md.Parse after defaults are reflected.
• Update README.md (page configuration section and the agent-facing guide) to document key.
• Add testdata/key.md (+ golden) to TestParse and a new TestValidateKeys covering: no keys, all unique, mixed keyed/unkeyed, empty-key collisions (not duplicates), and a duplicate-key error.

Verified with go build ./... and go test ./... (all packages pass).