feat(lint): enforce package architecture boundaries

[hansjm10]

Sequence

Order: 9 of 12
Phase: Phase 3 - Mechanical Architecture Guardrails
Design: docs/agent-first-workflow-design.md
Tracker: #908
Depends on: #916

Agent Role

Architecture Guard Agent

Scope

• Add mechanical package/domain boundary checks for the monorepo.
• Prefer ESLint rules or structural tests with remediation messages agents can act on.
• Document allowed and disallowed dependency edges before making checks blocking.

Context Packet

• docs/agent-first-workflow-design.md
• docs/agent-map.md if present
• packages/config-eslint/index.js
• eslint.config.mjs
• Package-level eslint.config.js files
• packages/core/README.md
• packages/shell-desktop/README.md

Acceptance Criteria

• The check prevents packages/core from importing shell, renderer implementations, docs, or tool packages.
• The check prevents renderer-contract from importing renderer implementations.
• The check prevents content schema/compiler packages from importing shell-desktop.
• Game/app-facing packages continue to avoid accidental @idle-engine/core/internals usage.
• Legitimate shell-desktop integration paths are documented and covered by tests or examples.
• Failure messages explain the preferred dependency direction or import surface.

Validation

• pnpm lint
• pnpm test:ci --filter @idle-engine/config-eslint if tests are added
• pnpm fast:check

Notes

• If current violations exist, either fix them in small scoped commits or start the rule in report/warn mode with explicit follow-up issues.

Refs #906