[Change] Doc Frontmatter Schema & Validation

[djm81]

User Story

As a SpecFact maintainer, I want documentation pages to declare ownership and tracked source paths in YAML frontmatter, with automated validation in pre-commit and Hatch, so docs stay aligned with code and rollout can expand safely.

OpenSpec Change

• Change folder: openspec/changes/doc-frontmatter-schema/
• Parent Feature: #356 — Documentation & Discrepancy Remediation
• Priority: P2
• Depends on: none (foundational for ci-docs-sync-check)
• Related: openspec/changes/ci-docs-sync-check/ (follow-up)

Scope

• scripts/check_doc_frontmatter.py + scripts/check-doc-frontmatter.py — validate doc_owner, tracks, last_reviewed, exemption fields
• Rollout via docs/.doc-frontmatter-enforced (expand paths over time); --all-docs for full-site check
• hatch run doc-frontmatter-check; wired into docs-validate and pre-commit (check-doc-frontmatter)
• Guide: docs/contributing/docs-sync.md; CONTRIBUTING updated; sample pages with full schema

Acceptance

• Enforced-path validation passes on CI; tests cover unit + integration scenarios
• openspec validate doc-frontmatter-schema --strict passes