docs: add CLAUDE.md documenting project structure and tests

CLAUDE.md

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+# CLAUDE.md
+
+This file orients contributors and AI agents working in this repository.
+
+## What this project is
+
+A Gradle plugin (`io.github.reqstool.gradle-plugin`) for
+[reqstool](https://github.com/reqstool/reqstool-client). It collects `@Requirements` and
+`@SVCs` annotations emitted by the reqstool annotation processor during compilation,
+combines them with a dataset (`requirements.yml`, etc.) and test result XML files, and
+packages everything into a `<name>-<version>-reqstool.zip` artifact that the reqstool CLI
+can analyze. Requires Java 21+.
+
+See `README.md` for the user-facing configuration reference (extension properties, task
+behavior, JUnit 5 parameterized test setup) and `CONTRIBUTING.md` for setup/DCO/commit
+conventions. This file focuses on internal structure, tests, and fixtures.
+
+## Project structure
+
+```
+src/main/java/io/github/reqstool/gradle/
+  RequirementsToolPlugin.java     # registers the assembleRequirements task, wires
+                                   # compileJava/build dependencies, configures publishing
+  RequirementsToolTask.java       # core logic: combine/merge annotation YAML, assemble zip
+  RequirementsToolExtension.java  # DSL + zero-config defaults
+
+src/test/java/io/github/reqstool/gradle/
+  RequirementsToolTaskTest.java   # unit tests (see below)
+
+src/test/resources/{yml,zip}/     # tracked reference fixtures (see "Fixtures" below)
+
+tests/fixtures/test_project/      # standalone Gradle project for manual end-to-end validation
+
+docs/modules/ROOT/                # Antora documentation source (published to reqstool.github.io)
+```
+
+The main task is `assembleRequirements` (group `build`). When the `java` plugin is applied,
+the plugin auto-wires it to depend on `compileJava` and every non-main source set's compile
+task, and makes `build` finalized by `assembleRequirements`.
+
+## Running tests
+
+```bash
+./gradlew test     # unit tests only
+./gradlew build    # compile, test, assemble plugin jar
+./gradlew format   # apply io.spring.javaformat formatting
+```
+
+## Unit tests — `RequirementsToolTaskTest`
+
+Uses `ProjectBuilder` with a `@TempDir` project directory and builds YAML in-memory via
+Jackson (`ObjectMapper` + `YAMLFactory`) — it does **not** read from `src/test/resources`.
+
+| Test | Validates |
+|---|---|
+| `testCombineOutput_bothEmpty` / `_withImplementations` / `_withTests` / `_withBoth` | `combineOutput()` produces a `requirement_annotations` node with `implementations`/`tests` keys present only when non-empty |
+| `testTaskConfiguration` | Extension defaults (`outputDirectory`, `datasetPath`, `skip`, `skipAssembleZipArtifact`, etc.) flow correctly into task properties |
+| `testSkipExecution` | `skip = true` causes `task.execute()` to no-op without throwing |
+| `testMergeTestNodes_mergesTwoFiles` | `mergeTestNodes()` unions SVC entries from multiple source-set `annotations.yml` files, merging arrays for keys present in both (e.g. `SVC_001` ends up with entries from both files) |
+| `testSetSvcsAnnotationsFilesMarksExplicit` | Setting `svcsAnnotationsFiles` explicitly marks the extension as no longer using auto-discovery |
+| `testDeprecatedSvcsAnnotationsFileSetter` | The deprecated singular `setSvcsAnnotationsFile()` still works and delegates to `svcsAnnotationsFiles` |
+| `testMissingRequirementsFile` | `task.execute()` throws an exception whose message mentions `requirements.yml` when the dataset directory doesn't contain it |
+
+## Fixtures
+
+### `src/test/resources/{yml,zip}/`
+
+Tracked example files showing the shape of annotation YAML (`requirements_annotations.yml`,
+`svcs_annotations.yml`, `combined_annotations.yml`, following the reqstool
+`annotations.schema.json`) and of an assembled zip's contents (`zip/` — dataset YAML files,
+combined `annotations.yml`, `surefire-reports/`, `failsafe-reports/`).
+
+**These are not currently loaded by `RequirementsToolTaskTest`** (which builds its test data
+inline) — treat them as reference examples of expected file formats, not as golden files
+with an automated regeneration step.
+
+### `tests/fixtures/test_project/` — manual end-to-end validation
+
+A minimal real Gradle project (`mypackage`) with three source sets, each producing one
+annotation that must survive the full pipeline:
+
+| Source set | Directory | Produces |
+|---|---|---|
+| `main` | `src/main/java` | `@Requirements` on `Hello.hello()` → `REQ_001` |
+| `test` | `src/test/java` | `@SVCs` on `HelloTest.testHello()` → `SVC_001` |
+| `integrationTest` | `src/integration-test/java` | `@SVCs` on `HelloIT.testHelloIntegration()` → `SVC_002` |
+
+Only source files, `build.gradle`, `settings.gradle`, and `docs/reqstool/*.yml` are tracked
+in git; `build/` and `.gradle/` are local build output and are gitignored.
+
+To validate (or to regenerate/inspect expected output after an intentional change to plugin
+behavior), run from `tests/fixtures/test_project/`:
+
+```bash
+gradle build
+ls build/reqstool/mypackage-0.1.0-reqstool.zip
+unzip -l build/reqstool/mypackage-0.1.0-reqstool.zip
+grep "SVC_" build/reqstool/annotations.yml   # expect both SVC_001 and SVC_002
+```
+
+Optionally, extract the zip and run `reqstool status local -p <extracted-dir>` (requires
+`pip install reqstool`) — expect `REQ_001` implemented and `SVC_001`/`SVC_002` both covered.
+See `tests/fixtures/test_project/README.md` for full expected output.