Add AGENTS.md file for agentic coding assistant steering (#3371)

Signed-off-by: Harkrishn Patro <bunty.hari@gmail.com>

AGENTS.md

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+# AGENTS.md
+
+## Scope
+- These instructions apply to the entire repository unless a deeper `AGENTS.md` overrides them.
+
+## Repo overview
+- This is the Valkey server codebase.
+- The main implementation lives under `src/`.
+- Unit tests live under `src/unit`
+- Integration tests live under `tests/`.
+- Top-level `Makefile` forwards most targets into `src/Makefile`.
+
+## Working guidelines
+- Keep changes minimal and easy to backport.
+- Match the style of the surrounding code instead of introducing new patterns.
+- Avoid unrelated refactors in the same change.
+
+## Build
+- Default build: `make`
+- Clean rebuild when build settings or bundled deps change: `make distclean && make`
+
+## Unit tests
+- Unit tests live under `src/unit/` and use GoogleTest (gtest/gmock).
+- Build and run all unit tests: `make -C src test-unit`
+- Unit tests cover data-structure and low-level logic changes.
+- A single test filter can be run with: `make -C src test-unit && ./src/unit/valkey-unit-gtests --gtest_filter='<TestSuite>.<TestName>'`
+
+## Integration tests
+- Integration tests live under `tests/` and are written in Tcl.
+- Run the full integration suite: `make test` (from the repo root).
+- Run a single test file: `./runtest --single <path/to/test.tcl>`
+- Additional specialized suites:
+  - Cluster tests: `./runtest-cluster`
+  - Sentinel tests: `./runtest-sentinel`
+  - Module API tests: `./runtest-moduleapi`
+- For targeted validation, run the smallest relevant test scope first before broader suites.
+
+## Code style
+- Follow the repository conventions described in `DEVELOPMENT_GUIDE.md`.
+- Most formatting is enforced by `clang-format`.
+- CI uses `clang-format-18` across `*.c`, `*.h`, `*.cpp`, and `*.hpp` files.
+- When touching C/C++ sources or headers, run `clang-format-18 -i` on the modified files before finalizing when the tool is available.
+- Use comments for non-obvious behavior and rationale, not for restating code.
+
+## Tests
+- Code changes should include relevant tests when the repo already has a matching test location.
+- Data-structure and low-level logic changes usually belong in `src/unit/` (C++ gtest).
+- End-to-end behavior changes usually belong in `tests/` (Tcl integration tests).
+- If behavior or commands change, check whether related documentation also needs updating.
+
+## Files to avoid touching unless required
+- Do not commit local runtime artifacts such as `dump.rdb`, `nodes.conf`, `*.log`, or ad hoc cluster directories unless the task explicitly requires them.
+- Treat vendored dependency code under `deps/` as special-case changes; modify it only when the task clearly requires it.
+
+## Pull Requests
+Always push to the user's fork. Never push to the upstream valkey-io/valkey repository. Never push directly to unstable. If a user fork does not exist, ask the contributor to create one.