docs(fixtures): add manual testing guide and AGENTS.md to test project

tests/fixtures/test_project/README.md

@@ -1,3 +1,71 @@
 # mypackage
 
-Minimal test project for manual validation of reqstool-python-poetry-plugin.
+Minimal test project for manual validation of `reqstool-python-poetry-plugin`.
+
+## Prerequisites
+
+A `.venv` must exist in the repository root with both the plugin and Poetry installed.
+If it is missing, recreate it from the repository root:
+
+```bash
+python3.13 -m venv .venv
+.venv/bin/pip install -e .
+.venv/bin/pip install poetry
+```
+
+Point Poetry at Python 3.13 (once per machine):
+
+```bash
+cd tests/fixtures/test_project
+../../../.venv/bin/poetry env use python3.13
+```
+
+## Validation
+
+Run all commands from `tests/fixtures/test_project/`.
+
+### 1 — Install
+
+```bash
+../../../.venv/bin/poetry install
+```
+
+Expected: plugin prints its version, then adds three entries to `pyproject.toml` under `[tool.poetry]`:
+
+```toml
+include = ["reqstool_config.yml", "docs/reqstool/**/*", "build/reqstool/**/*"]
+```
+
+### 2 — Build
+
+```bash
+../../../.venv/bin/poetry build
+```
+
+Expected output (in order):
+1. `[reqstool] plugin version <x.y.z>`
+2. `[reqstool] added to reqstool_config.yml: docs/reqstool/requirements.yml`
+3. `[reqstool] added to reqstool_config.yml: build/reqstool/annotations.yml`
+4. `[reqstool] Created reqstool_config.yml in project root`
+5. Poetry builds sdist + wheel
+6. `[reqstool] Removed reqstool_config.yml from project root`
+
+### 3 — Verify
+
+```bash
+# annotations.yml must exist
+test -f build/reqstool/annotations.yml && echo "OK: annotations.yml"
+
+# reqstool_config.yml must be gone from project root after build
+test ! -f reqstool_config.yml && echo "OK: reqstool_config.yml cleaned up"
+
+# sdist must contain all reqstool files
+tar -tzf dist/mypackage-0.1.0.tar.gz | sort
+```
+
+Expected entries in the sdist:
+- `mypackage-0.1.0/reqstool_config.yml`
+- `mypackage-0.1.0/build/reqstool/annotations.yml`
+- `mypackage-0.1.0/docs/reqstool/requirements.yml`
+
+`annotations.yml` must contain `REQ_001` mapped to `src.mypackage.main.hello`.