chore: add AGENTS.md

[ccharly]

Adding an AGENTS.md file based on the current codebase style and architecture.

Took inspiration from the existing one on the extension:

• https://github.com/MetaMask/metamask-extension/blob/main/AGENTS.md

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Note

Adds a comprehensive AGENTS.md documenting standards and workflows for the TypeScript monorepo.

• Centralizes rules (TypeScript-only, no any, colocated tests), linting/formatting, dependency practices
• Lists common commands (yarn build, yarn test, yarn lint:fix, release flow) and step-by-step workflows (feature, bugfix, new package, release)
• Outlines project structure, package roles, where to place code, and architecture/testing/type guidelines
• Includes decision trees, troubleshooting, and pre-completion checklists to ensure quality and consistency

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