Add AGENTS.md and design-intent docs for chart components

[pppp606]

Motivation

• Provide a short developer guide for AI agents working on this repository to standardize scope and operational rules.
• Clarify the behavioral/design intent of terminal chart components so changes prioritize narrow-terminal readability and low-density output.
• Surface library-selection guidance in the README for AI-assisted decision making about using this package.

Description

• Add AGENTS.md containing scope, development priorities, operational rules, and a validation checklist for repo work by agents.
• Add an "AI Library Selection Notes" section to README.md explaining when ink-chart is appropriate and when to prefer other tools.
• Insert design-intent JSDoc/comments into BarChart.tsx, LineGraph.tsx, Sparkline.tsx, and StackedBarChart.tsx to document tradeoffs (readability, density, narrow-terminal behavior) and guide future changes.

Testing

• Ran the TypeScript typecheck via npm run typecheck and it completed successfully.
• Executed the existing unit test suite with npm test and all tests passed.

---

Codex Task