chore: add AGENTS.md (#324)

Author: strantalis

AGENTS.md

@@ -0,0 +1,40 @@
+# Repository Guidelines
+
+## Project Structure & Modules
+
+- Multi-module Maven repo (root `pom.xml`) with three modules:
+  - `sdk/`: the OpenTDF Java SDK (`sdk/src/main/java`, `sdk/src/main/kotlin`, tests in `sdk/src/test/java`)
+  - `cmdline/`: shaded CLI jar (`cmdline/src/main/java`, resources in `cmdline/src/main/resources`)
+  - `examples/`: runnable examples (`examples/src/main/java`)
+- Protobuf/gRPC sources are generated during the build into `sdk/target/generated-sources` (do not hand-edit generated code).
+
+## Build, Test, and Dev Commands
+
+- Build everything (runs codegen): `mvn clean install`
+- Fast local build (skip tests): `mvn clean install -DskipTests`
+- SDK unit tests: `mvn -pl sdk test`
+- Verify like CI (coverage profile): `mvn clean verify -P coverage`
+- Build CLI jar: `mvn -pl cmdline -am package` (output: `cmdline/target/cmdline.jar`)
+- Run an example via exec plugin: `mvn -pl examples exec:java@EncryptExample`
+
+## Coding Style & Naming
+
+- Target runtime: Java 11 (`maven.compiler.release=11`). CI runs on newer JDKs but compiles to 11 bytecode.
+- No repo-wide formatter is enforced; match surrounding style (4-space indent, no tabs, consistent import grouping).
+- Packages use `io.opentdf.platform...`; keep public SDK APIs stable and additive where possible.
+
+## Testing Guidelines
+
+- Tests use JUnit (Jupiter) plus Mockito/AssertJ; place new tests under `sdk/src/test/java` and name `*Test`.
+- Prefer focused unit tests over integration tests; keep network calls mocked (e.g., `MockWebServer`).
+
+## Commit & Pull Request Guidelines
+
+- Use Conventional Commits (enforced on PR titles): `feat(sdk): ...`, `fix(cmdline): ...`, `chore(docs): ...` (use `!` for breaking changes).
+- DCO sign-off is required: `git commit -s`.
+- PRs should include: summary, rationale, test plan/commands run, and any public API notes.
+- Build/dependency files (e.g., `pom.xml`, `sdk/pom.xml`) are code-owned and may require security/architecture review.
+
+## Security & Configuration Tips
+
+- Builds require the `buf` CLI (see `sdk/buf.yaml` / `sdk/buf.gen.yaml`). Authenticate to avoid rate limits; CI uses `BUF_INPUT_HTTPS_USERNAME`/`BUF_INPUT_HTTPS_PASSWORD`. Never commit tokens.