docs: add AGENTS.md for project overview and working rules

[Frozen]

This pull request adds a new AGENTS.md file at the repository root, providing clear guidelines for contributors and automated agents. The document outlines the project's scope, working rules, recommended commands, build notes, and documentation standards to ensure consistency and security across contributions.

Contributor guidelines:

• Added AGENTS.md with repository-wide instructions on code changes, including scope, formatting, testing, and security-sensitive areas.
• Documented useful Go and Makefile commands for building, testing, and maintaining the project.
• Included best practices for running checks, build steps, and documentation updates before completing a pull request.
• Specified requirements for pull request descriptions, including a [Test] section summarizing validation steps and results.

[Frozen]

@copilot AGENTS.md contains actual information?

[Copilot]

@copilot AGENTS.md contains actual information?

Yes — the file contains real, repository-specific information:

• Project overview: describes Harmony as a Go blockchain node, its main executables (node + bootnode), and the specific Go version (1.24.2) and BLS dependency (github.com/herumi/bls-eth-go-binary) used.
• Working rules: concrete conventions like formatting with gofmt/goimports, security-sensitive areas (consensus, cryptography, staking, slashing), and restrictions like not editing generated files directly.
• Useful commands: exact make targets (make exe, make go-vet, make go-test, make protofiles, make debug, etc.) and targeted go test/go vet patterns.
• Build notes: guidance on fetching the BLS dependency (go mod download or make libs) and caveats around Docker/native libraries for full test runs.
• Documentation and PR standards: requirement for a [Test] section in PR descriptions.

The content is intended to guide both human contributors and AI coding agents working in this repository.

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