Rename AGENT.md to AGENTS.md and add symlinks for different AI agents

[skarpovdev]

Motivation and Context

We get vibe-coded PRs from time to time where the quality of the code would have been higher if the author used our AGENTS.md, which is not obvious. This PR makes it easy for different AI agents to automatically find our AGENTS.md by using symlinks from the files they are loading by default.

Breaking Changes

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Type of the changes

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Tests improvement
• Refactoring
• CI/CD changes
• Dependencies update

Checklist

• The pull request has a description of the proposed change
• I read the Contributing Guidelines before opening the pull request
• The pull request uses develop as the base branch
• Tests for the changes have been added
• All new and existing tests passed

Additional steps for pull requests adding a new feature

• An issue describing the proposed change exists
• The pull request includes a link to the issue
• The change was discussed and approved in the issue
• Docs have been added / updated

[antoniibelyshev]

Thank you! LGTM