fix: align REQUIRED_FRONTMATTER_KEYS with OKF spec §4.1

[tcconnally]

Problem

REQUIRED_FRONTMATTER_KEYS in document.py was set to ("type", "title", "description", "timestamp"), treating four fields as required. However, OKF v0.1 §4.1 states that only type is REQUIRED. title, description, and timestamp are "Recommended" — not required.

This means the reference implementation was rejecting valid OKF bundles that conform to the spec. For example, a concept document with only type and tags (both spec-valid) would fail validate().

Fix

• Changed REQUIRED_FRONTMATTER_KEYS to ("type",) — only type is required per spec.
• Updated test_validate_rejects_missing_required_keys → test_validate_rejects_missing_type to test that only missing type triggers validation failure.
• Added test_validate_accepts_minimal_frontmatter to verify that {"type": "X"} passes validation (spec-minimal bundle).

Verification

All 6 document tests pass:

tests/test_document.py::test_roundtrip_preserves_frontmatter_and_body PASSED
tests/test_document.py::test_parse_no_frontmatter_treats_all_as_body PASSED
tests/test_document.py::test_unterminated_frontmatter_raises PASSED
tests/test_document.py::test_validate_rejects_missing_type PASSED
tests/test_document.py::test_validate_accepts_minimal_frontmatter PASSED
tests/test_document.py::test_validate_accepts_full_frontmatter PASSED

Impact

This is a conformance fix. Without it, any downstream producer that emits spec-minimal bundles (type-only frontmatter) would get false validation failures from the reference validator. This is especially important for programmatic producers that may not have human-readable title/description fields available at generation time.

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[tcconnally]

Friendly ping — CLA signed and checks passing. Happy to iterate on any feedback.