docs: add AGENTS.md agent-onboarding guide

AGENTS.md

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+# AGENTS.md — determa-state-python
+
+Guidance for AI/coding agents working in this repository. (Tool-agnostic; not specific to any one assistant.)
+
+## What this repo is
+The **Python reference implementation** of Determa State. Distribution name
+**`determa-state`**; import name **`determa.state`** (a PEP 420 namespace package — there is
+**no** `src/determa/__init__.py`, so it coexists with the `determa` launcher package). It is
+correct **iff** it passes the conformance suite.
+
+Layout:
+- `src/determa/state/` — the engine package (`__about__.py` is the single version source).
+- `tests/` — the implementation's own **unit tests** (hermetic, offline).
+- `conformance/` — a **black-box harness** that fetches `determa-state-conformance` at the
+  tag matching this package's version into `.cache/` (override with `DETERMA_CONFORMANCE_DIR`;
+  spec schema override `DETERMA_SPEC_DIR`).
+- `.github/workflows/` — `test.yml` (CI gate) and `release.yml` (tag → PyPI).
+
+## Determa in one paragraph
+**Determa** is a family for defining/running well-specified, verifiable behavior. **Determa
+State** is a language-agnostic **statechart engine** (Harel/UML lineage, PSiCC RTC): one
+YAML/JSON machine runs identically under any implementation, validated against a shared
+conformance suite. Guards/action values are **CEL** (via the `cel-python`/`celpy` package,
+lazily imported so the CLI starts fast). An umbrella `determa` launcher dispatches
+`determa <product> …` → `determa-<product>` on PATH; this package also installs a
+`determa-state-python` alias for explicit implementation selection.
+
+## Repositories (org `fruwehq`, local folders `~/src/personal/`)
+| Repo | Role |
+|---|---|
+| determa-state-spec | normative prose spec + schema. No CI. |
+| determa-state-conformance | the conformance suite (arbiter). No CI. |
+| **determa-state-python** (this) | Python impl — `determa-state` / `determa.state`. |
+| determa-state-rust | Rust impl — crate `determa-state`. |
+| determa | umbrella launcher (`python/`, `rust/`, `node/`). |
+
+## Working rules (every Determa repo)
+- **One issue → one PR**, branch → PR → **squash-merge**, linear history, resolve threads; `main` is protected (**and requires branches be up-to-date** — after a merge moves `main`, update other open PRs, which re-runs CI, before merging).
+- **No AI/assistant attribution** anywhere (commits, PRs, comments, docs).
+- **Conformance-first:** spec text → conformance case → this impl. Don't diverge from the pinned suite.
+- **Synchronized SemVer** with spec + rust (currently **0.0.6**); bump `src/determa/state/__about__.py`.
+- **No abbreviations** in JSON output / public identifiers (`definition` not `def`). Kept for now: `config`, machine-keywords (`esvs`, …), snapshot `def_id`/`def_version`, `spawn.def`.
+
+## Gates (run before requesting review — this is the CI gate)
+```sh
+pip install -e '.[dev]'
+ruff check .
+mypy src/determa
+pytest -q                 # unit tests (hermetic, offline) — CI job "test (ubuntu-24.04)"
+pytest conformance        # conformance suite (network, or DETERMA_CONFORMANCE_DIR) — CI job "conformance"
+# convenience: `make check` (gate) and `make conformance`
+```
+Keep CEL/`jsonschema` imports lazy (they dominate startup); unit tests must not touch the network.
+
+## Releasing
+Tag `vX.Y.Z` → `release.yml` builds sdist+wheel and publishes to **PyPI via Trusted
+Publishing (OIDC)** — **gated on the `pypi` GitHub Environment (manual approval)**. The
+PyPI project name is `determa-state`. **A tag publishes**, so only tag when you intend to
+release. After a spec release, the conformance fetch auto-targets the new `v{version}` tag.
+
+## Pointers
+- Library API (SPEC §2): `Host`, `Instance`, `load_definitions` (accepts YAML **or** a dict/mapping), `validate`, etc. — see `README.md` and `tests/test_library_api.py`.
+- CLI (SPEC §13/§14): `src/determa/state/cli.py`. Spec: `determa-state-spec/SPEC.md`.