docs(readme): add missing package READMEs

[hansjm10]

Sequence

Order: 6 of 12
Phase: Phase 2 - Package Legibility and Hygiene
Design: docs/agent-first-workflow-design.md
Tracker: #908
Depends on: #913

Agent Role

Docs Agent

Scope

• Add concise README entry points for workspace packages and tools that currently lack one.
• Use a consistent structure: purpose, entry points, key invariants, commands, related docs, and validation guidance.
• Keep README content factual and aligned with package scripts and exports.

Context Packet

• docs/agent-map.md
• docs/agent-first-workflow-design.md
• package.json
• pnpm-workspace.yaml
• packages/*/package.json
• tools/*/package.json
• Existing package READMEs such as packages/core/README.md and packages/shell-desktop/README.md

Acceptance Criteria

• README files exist for packages/content-compiler, packages/content-schema, packages/controls, packages/renderer-contract, packages/renderer-debug, tools/content-schema-cli, tools/coverage-report, and tools/runtime-sim unless a package no longer exists.
• Each README lists package purpose, public entry points or CLI commands, invariants, related docs, and validation commands.
• README claims match package scripts and source layout.
• docs/agent-map.md links to the new READMEs where appropriate.

Validation

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

Notes

• Avoid changing package APIs or scripts in this issue unless a README reveals a clearly broken command.

Refs #906