[Task] Document issue and PR metadata automation contract #507

Deliverable

Document the canonical issue/PR metadata contract based on live labeling automation, template guardrails, and project field sync.

Scope Boundary

• In scope: contributor-facing contract and reference links.
• Out of scope: introducing new label families or required fields.

Acceptance Checklist

• One-hot expectations for status:*, priority:*, and type:* are explicit.
• Changelog meta label policy uses canonical names.
• PR/issue template guidance references live guardrails.
• Cross-links to canonical spec and current workflow/config files are present.

Branch Prefix Expectation

• Use docs/.

Validation Commands

npx markdownlint-cli2 "docs/PR_LABELS.md" "docs/LABEL_STRATEGY.md" "docs/GITHUB_PROJECT_OPERATIONS_SPEC.md"
node scripts/agents/includes/check-template-labels.js

Touched Paths

• docs/PR_LABELS.md
• docs/LABEL_STRATEGY.md
• docs/GITHUB_PROJECT_OPERATIONS_SPEC.md
• .github/labels.yml
• .github/labeler.yml