chore: add AGENTS.md (#436)

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

<!-- CURSOR_SUMMARY -->
---

> [!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
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
3d9759c. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Mathieu Artu <mathieu.artu@gmail.com>

Author: ccharly