docs: add copilot-instructions.md for AI coding agent onboarding (#6112)

* docs: improve copilot-instructions.md with specific style guide reference
---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: romange <3674760+romange@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -0,0 +1,187 @@
+# Copilot Coding Agent Instructions for Dragonfly
+
+## Repository Overview
+
+Dragonfly is a high-performance, Redis and Memcached compatible in-memory data store written in C++17. It uses a shared-nothing architecture for efficient multi-threaded operation, delivering significantly higher throughput than traditional single-threaded Redis implementations.
+
+**Key characteristics:**
+- Large C++ codebase (~200k+ LOC) with Python integration tests
+- Uses CMake/Ninja build system with the `helio` submodule for I/O and threading
+- Targets Linux (kernel 5.11+ recommended), with io_uring and epoll support
+- Requires signed commits with conventional commit messages
+
+## Repository Structure
+
+```
+src/                    # Main C++ source code
+├── server/             # Core server implementation (main entry: dfly_main.cc)
+├── core/               # Core data structures
+├── facade/             # Network and command handling
+├── redis/              # Redis protocol implementation
+helio/                  # Git submodule: I/O and threading library (must be initialized)
+tests/dragonfly/        # Python pytest integration/regression tests
+contrib/                # Docker configs, Helm charts, utility scripts
+docs/                   # Documentation including build-from-source.md
+tools/                  # Benchmarking and utility tools
+```
+
+## Build Instructions
+
+### Prerequisites (Ubuntu/Debian)
+
+```bash
+sudo apt install ninja-build libunwind-dev libboost-context-dev libssl-dev \
+     autoconf-archive libtool cmake g++ bison zlib1g-dev
+```
+
+### Alternative: Build with Docker (all prerequisites included)
+
+Use pre-built development containers that have all dependencies installed:
+
+```bash
+docker run -it --rm -v $(pwd):/src -w /src ghcr.io/romange/ubuntu-dev:24 bash
+# Or use Ubuntu 22.04:
+docker run -it --rm -v $(pwd):/src -w /src ghcr.io/romange/ubuntu-dev:22 bash
+```
+
+### Initialize Submodules (REQUIRED)
+
+```bash
+git submodule update --init --recursive
+```
+
+### Configure and Build
+
+**Debug build (recommended for development):**
+```bash
+./helio/blaze.sh                    # Creates build-dbg/
+cd build-dbg && ninja dragonfly
+```
+
+**Release build:**
+```bash
+./helio/blaze.sh -release           # Creates build-opt/
+cd build-opt && ninja dragonfly
+```
+
+**Minimal debug build (faster compilation):**
+```bash
+./helio/blaze.sh -DWITH_GPERF=OFF -DWITH_AWS=OFF -DWITH_GCP=OFF \
+    -DWITH_TIERING=OFF -DWITH_SEARCH=OFF -DWITH_COLLECTION_CMDS=OFF \
+    -DWITH_EXTENSION_CMDS=OFF
+```
+
+### Build Specific Test
+
+```bash
+cd build-dbg && ninja generic_family_test
+./generic_family_test
+```
+
+## Testing
+
+### C++ Unit Tests
+
+```bash
+cd build-dbg
+ninja src/all                       # Build all targets in src/
+ctest -V -L DFLY                    # Run all tests labeled DFLY
+```
+
+Individual test execution with environment variables:
+```bash
+GLOG_alsologtostderr=1 FLAGS_fiber_safety_margin=4096 timeout 20m ctest -V -L DFLY
+```
+
+### Python Regression Tests
+
+```bash
+cd tests
+pip3 install -r dragonfly/requirements.txt
+export DRAGONFLY_PATH="../build-dbg/dragonfly"
+pytest dragonfly/ --log-cli-level=INFO
+```
+
+**Test markers:**
+- `slow`: Long-running tests
+- `opt_only`: Only run on release builds
+- `debug_only`: Only run on debug builds
+- `exclude_epoll`: Skip when using epoll
+
+### Helm Chart Tests
+
+```bash
+cd contrib/charts/dragonfly
+go test -v ./... -update            # Update golden files if needed
+```
+
+## Code Style and Pre-commit Hooks
+
+**ALWAYS install pre-commit hooks before making changes:**
+
+```bash
+pipx install pre-commit clang-format black
+pre-commit install
+```
+
+**Style guidelines:**
+- C++: [Google C++ Style Guide (2020 version)](https://github.com/google/styleguide/blob/505ba68c74eb97e6966f60907ce893001bedc706/cppguide.html), clang-format v14.0.6, 100 column limit
+- Python: Black formatter, 100 character line length
+- Commits: Conventional Commits format, must be signed (`git commit -s`)
+
+**Commit message format:**
+```
+<type>(<scope>): <description>
+
+Valid types: feat, fix, build, chore, ci, docs, perf, refactor, revert, style, test
+```
+
+Example: `fix(server): correct memory leak in db_slice #123`
+
+## CI/CD Pipeline
+
+The main CI workflow (`.github/workflows/ci.yml`) runs on PRs to main:
+
+1. **pre-commit**: Validates formatting and commit messages
+2. **build**: Tests on ghcr.io/romange containers (ubuntu-dev:20, ubuntu-dev:24, alpine-dev)
+   - Debug and Release configurations
+   - GCC and Clang compilers
+   - With/without ASAN/UBSAN sanitizers
+3. **Unit tests**: IoUring mode, Epoll mode, Cluster mode variants
+4. **Regression tests**: Python pytest suite
+5. **Helm chart tests**: Lint and installation tests
+
+## Key Files for Reference
+
+| Purpose | File |
+|---------|------|
+| Main entry point | `src/server/dfly_main.cc` |
+| Server configuration | `src/server/main_service.cc` |
+| CMake root | `CMakeLists.txt` |
+| Build script | `helio/blaze.sh` |
+| CI workflow | `.github/workflows/ci.yml` |
+| Pre-commit config | `.pre-commit-config.yaml` |
+| C++ formatting | `.clang-format` |
+| Python formatting | `pyproject.toml` |
+| Pytest config | `tests/pytest.ini` |
+| Test requirements | `tests/dragonfly/requirements.txt` |
+
+## Common Pitfalls
+
+1. **Submodule not initialized**: Always run `git submodule update --init --recursive` after cloning
+2. **Missing pre-commit hooks**: Run `pre-commit install` to avoid commit failures
+3. **Unsigned commits**: Always use `git commit -s` for signed commits
+4. **Build directory confusion**: Debug builds go to `build-dbg/`, release to `build-opt/`
+5. **Test timeouts**: Use `timeout` command for long-running tests to avoid hangs
+
+## Validation Checklist
+
+Before submitting changes:
+
+1. ✅ Code compiles: `cd build-dbg && ninja dragonfly`
+2. ✅ Unit tests pass: `ctest -V -L DFLY`
+3. ✅ Pre-commit checks pass: `pre-commit run` (runs on staged files) or `pre-commit run --files path/to/file1 path/to/file2`
+4. ✅ Commits are signed and follow conventional format
+5. ✅ For Python changes: `pytest tests/dragonfly/ -k <test_name>`
+
+Trust these instructions. Only search for additional information if these instructions are incomplete or produce errors.