Add root-level AGENTS.md for AI coding assistants

AGENTS.md

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+# AGENTS.md
+
+Guidance for AI coding assistants (Claude Code, Codex, Copilot agent mode, Cursor, Gemini CLI, Jules, Aider, ...)
+working in this repository. See [CONTRIBUTING.md](CONTRIBUTING.md) for the full human contributor guide and
+[the tool-assisted contribution policy](CONTRIBUTING.md#tool-assisted-contribution-policy) for what is and isn't
+acceptable when using these tools — this file is about *how the codebase works*, not *whether AI assistance is
+allowed*.
+
+## Project overview
+
+MONAI is a PyTorch-based, open-source framework for deep learning in healthcare imaging. See
+[pyproject.toml](pyproject.toml) and [setup.py](setup.py) for the supported Python and PyTorch versions.
+
+## Repository map
+
+| Path | Purpose |
+|------|---------|
+| `monai/` | the library itself: transforms, networks, losses, metrics, data, bundle, apps, engines, etc. |
+| `tests/` | unit and integration tests, mirroring `monai/`'s layout |
+| `docs/` | Sphinx documentation sources (`docs/source/*.rst`, `*.md`) |
+| `runtests.sh` | the test and style-check entry point used in CI and locally |
+
+## Required workflow
+
+- Pull requests target the `dev` branch, not `main`.
+- Every commit needs a DCO sign-off (`git commit -s`, i.e. a `Signed-off-by:` trailer). The DCO app fails PRs
+  with commits missing it; see [Signing your work](CONTRIBUTING.md#signing-your-work).
+- Format and lint with `./runtests.sh --autofix` (runs `isort`, `black` and `ruff check --fix`) before
+  committing. If pre-commit hooks are installed (`./runtests.sh --setup`), the same checks also run on every
+  `git commit`.
+
+## Code conventions
+
+- Optional third-party dependencies are imported lazily via `monai.utils.optional_import`, not a bare
+  `import`, so the package stays usable without every extra installed.
+- A new public API needs a corresponding entry in `docs/source/*.rst` (or the matching `.md` file for guides),
+  not just a docstring.
+- A slow test is decorated with `skip_if_quick` (`tests/test_utils.py`); a test module that needs an optional
+  dependency is added to `exclude_cases` in `tests/min_tests.py`.
+
+## What stays out of this file
+
+Package-specific traps (e.g. for `monai/transforms`, `monai/networks`, `monai/bundle`) are intentionally not
+covered here — this root file only carries rules that apply repo-wide. Nested, package-level `AGENTS.md` files
+can follow in later PRs if the maintainers want that detail; see the discussion on #9199.