test(docs): enforce doc index coverage

[hansjm10]

Sequence

Order: 4 of 12
Phase: Phase 1 - Navigation and Freshness
Design: docs/agent-first-workflow-design.md
Tracker: #908
Depends on: #911

Agent Role

Architecture Guard Agent

Scope

• Add a docs check that detects top-level docs/*.md files missing from Docusaurus navigation or an approved explicit state.
• Define the approved states for discoverable, archived, and hidden docs.
• Wire the check into the docs test path after current repository docs are classified.
• Produce clear, agent-actionable failure messages listing missing docs and remediation options.

Context Packet

• docs/agent-first-workflow-design.md
• packages/docs/sidebars.ts
• packages/docs/scripts/test-docs.mjs
• packages/docs/package.json
• docs/design-document-template.md

Acceptance Criteria

• The docs test reports every unclassified top-level docs/*.md file.
• Docs may pass by being included in the sidebar or by carrying approved archived/hidden metadata.
• The check includes remediation text suitable for agents.
• The repository passes the new check after necessary classifications or sidebar updates.
• The rule does not require generated coverage/performance pages to be manually edited.

Validation

• pnpm --filter @idle-engine/docs test:ci
• pnpm docs:build

Notes

• Prefer a small script/test over broad Docusaurus config changes.
• If many docs need classification, make minimal metadata/sidebar changes and defer content rewrites.

Refs #906