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

AGENTS.md

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+# AGENTS.md
+
+## Scope
+
+These instructions apply to the entire repository. More specific `AGENTS.md`
+files in subdirectories override them for files in those directories.
+
+## Project overview
+
+Harmony is a Go blockchain node implementation. The main executables are the
+Harmony node and bootnode. The repository also contains consensus, networking,
+staking, RPC, Rosetta, and local-network test code.
+
+The project uses Go 1.24.2. BLS support comes from the official
+`github.com/herumi/bls-eth-go-binary` Go module with prebuilt static libraries;
+separate MCL/BLS checkouts and custom native-library build flags are not needed.
+
+## Working rules
+
+- Keep changes focused on the requested task; do not modify unrelated files.
+- Preserve existing public behavior unless the task explicitly changes it.
+- Follow Effective Go and the conventions already used in the edited package.
+- Format Go files with `gofmt` and `goimports`.
+- Add or update tests for behavioral changes and bug fixes.
+- Do not edit generated files directly. Change their source and use the
+  repository's generation command.
+- Never commit secrets, validator keys, local databases, logs, build outputs,
+  or files created by localnet runs.
+- Treat consensus, cryptography, staking, slashing, and state-transition
+  changes as security-sensitive. Check boundary conditions and compatibility
+  carefully.
+
+## Useful commands
+
+Run the narrowest relevant checks first:
+
+```bash
+go test ./path/to/changed/package
+go test -run TestName ./path/to/changed/package
+go vet ./path/to/changed/package
+```
+
+Repository-level commands:
+
+```bash
+make exe          # build the Harmony and bootnode executables
+make go-vet       # run go vet for all packages
+make go-test      # run all Go tests with vet and the race detector
+make test-go      # run the Docker-based Go CI checks
+make protofiles   # regenerate protobuf-derived Go code
+make debug        # build and start a local development network
+make debug-kill   # stop the local development network
+```
+
+Before reporting completion:
+
+1. Run `gofmt`/`goimports` on changed Go files.
+2. Run targeted tests for the changed packages.
+3. Run broader checks when practical and state clearly which checks were not
+   run or could not run.
+4. Review `git diff --check` and the final diff for accidental changes.
+
+## Build notes
+
+- Prefer the Makefile and repository scripts over ad hoc build commands.
+- Use `go mod download` or `make libs` to fetch the BLS dependency. Do not
+  reintroduce sibling MCL/BLS checkouts or the removed native build-flag setup.
+- Full test and localnet commands may require Docker, native libraries, open
+  ports, and substantially more time than targeted package tests.
+- Do not run cleanup targets or remove localnet data unless the user explicitly
+  asks for cleanup.
+
+## Documentation and pull requests
+
+- Update documentation when configuration, commands, APIs, or operator-visible
+  behavior changes.
+- In the final report, summarize the change and list the exact validation
+  commands and results.
+- Pull request descriptions should include a `[Test]` section describing the
+  checks performed.