chore(agents): add atomic-unit-testing Agent Skill

[fbeaudoincoveo]

Context

This PR is a follow up to:

• chore: rework AGENTS.md files #7133
• chore(agents): simplify AGENTS.md #7180

It's the initial step in getting rid of all .instructions.md files and prompt.md files in favor of tool-agnostic open standards (AGENTS.md and Agent Skills).

What's Added

A new atomic-unit-testing Agent Skill that will be used by AI agents when writing, modifying, or reviewing unit tests, debugging test failures, or when the user mentions testing, spec files, Vitest, or test coverage in the Atomic package.

The Agent Skill is designed with progressive disclosure based on tested entity type.

It should be easy to extend it to encompass other ui-kit packages (in particular headless) in the near future. Of course we'll need to rename and rework the skill a little, but I think the core is good and generic enough to cover any package that uses Vitest for unit testing (i.e., everything except Quantic I think).

What's Removed

• All informations in .instructions.md files that related to Atomc unit tests
• All .prompt.md files (they all either related to Atomic unit test creation or to Stencil -> Lit migration)

Next Steps

• Continue getting rid of remaining .instructions.md files (immediate next step will be to add a managing-atomic-components Agent Skill, which will allow us to get rid of the atomic.instructions.md file altogether).
• Add assuming-personas Agent Skill: this one will enable agents to pick the best expert persona for the job at hand.
• Remove all .agent.md files (we will no longer need them with the assuming-personas Agent Skill)
• Rename .claude folder to .agents, update opencode.json config, and update all references in AGENTS.md and Agent Skills

https://coveord.atlassian.net/browse/KIT-5489

[github-actions]

⚠️ Secrets Reference Detected in Workflow Changes

This pull request modifies GitHub Actions workflow files and includes references to secrets.

Please ensure:

• The secret is properly configured in the repository/organization settings
• The secret name follows naming conventions
• The secret is not being exposed in logs or outputs
• Access to the secret is appropriately scoped

Why am I seeing this?

This comment is automatically added when workflow changes include the word secrets to remind reviewers to verify proper secret handling.

[github-actions]

🔗 Scratch Orgs ready to test this PR:

• ✅ LWS Enabled: Open Org
• ✅ LWS Disabled: Open Org