docs: add AGENTS.md files for AI code assistants

[dmmulroy]

Summary

Adds AGENTS.md files throughout the monorepo to provide context for AI coding assistants (Cursor, Claude, Copilot, etc).

Changes

• Root AGENTS.md - monorepo overview, structure, conventions, commands
• packages/effect/AGENTS.md - core package types and patterns
• packages/effect/src/internal/AGENTS.md - internal implementation guidance
• packages/platform/AGENTS.md - runtime abstraction patterns
• packages/sql/AGENTS.md - SQL package usage
• packages/cli/AGENTS.md - CLI framework patterns
• packages/ai/AGENTS.md - AI provider integrations

These files help AI assistants understand Effect conventions, anti-patterns, and where to look for specific functionality.

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Latest commit: 7b4d195

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