Add AGENTS.md (#349)

Introduce `AGENTS.md` documenting repository layout, build/test
commands, API compatibility checks, code conventions, logging/error
handling guidance, and the PR/review checklist for the Kotlin MCP SDK.

AGENTS.md

@@ -0,0 +1,95 @@
+# kotlin-mcp-sdk
+
+MCP Kotlin SDK — Kotlin Multiplatform implementation of the Model Context Protocol.
+
+## Repository Layout
+- `kotlin-sdk-core`: Core protocol types, transport abstractions, WebSocket implementation
+- `kotlin-sdk-client`: Client transports (STDIO, SSE, StreamableHttp, WebSocket)
+- `kotlin-sdk-server`: Server transports + Ktor integration (STDIO, SSE, WebSocket)
+- `kotlin-sdk`: Umbrella module that aggregates all three above
+- `kotlin-sdk-test`: Shared test utilities
+- `samples/`: Three sample projects (composite builds)
+
+## General Guidance
+- Avoid changing public API signatures. Run `./gradlew apiCheck` before every commit.
+- **Explicit API mode** is strict: all public APIs must have explicit visibility modifiers and return types.
+- Anything under an `internal` directory is not part of the public API and may change freely.
+- Package structure follows: `io.modelcontextprotocol.kotlin.sdk.*`
+- The SDK targets Kotlin 2.2+ and JVM 1.8+ as minimum.
+
+## Building and Testing
+1. Build the project:
+   ```bash
+   ./gradlew build
+   ```
+2. Run tests. To save time, run only the module you changed:
+   ```bash
+   ./gradlew :kotlin-sdk-core:test
+   ./gradlew :kotlin-sdk-client:test
+   ./gradlew :kotlin-sdk-server:test
+   ```
+3. For platform-specific tests:
+   ```bash
+   ./gradlew jsTest
+   ./gradlew wasmJsTest
+   ```
+4. Check binary compatibility (required before commit):
+   ```bash
+   ./gradlew apiCheck
+   ```
+5. If you intentionally changed public APIs, update the API dump:
+   ```bash
+   ./gradlew apiDump
+   ```
+
+## Tests
+- All tests for each module are located in `src/commonTest/kotlin/io/modelcontextprotocol/kotlin/sdk/`
+- Platform-specific tests go in `src/jvmTest/`, `src/jsTest/`, etc.
+- Use Kotest assertions (`shouldBe`, `shouldContain`, etc.) for readable test failures.
+- Use `shouldMatchJson` from Kotest for JSON validation.
+- Mock Ktor HTTP clients using `MockEngine` from `io.ktor:ktor-client-mock`.
+- Always add tests for new features or bug fixes, even if not explicitly requested.
+
+## Code Conventions
+
+### Multiplatform Patterns
+- Use `expect`/`actual` pattern for platform-specific implementations in `utils.*` files.
+- Test changes on JVM first, then verify platform-specific behavior if needed.
+- Supported targets: JVM (1.8+), JS/Wasm, iOS, watchOS, tvOS.
+
+### Serialization
+- Use Kotlinx Serialization with explicit `@Serializable` annotations.
+- Custom serializers should be registered in the companion object.
+- JSON config is defined in `McpJson.kt` — use it consistently.
+
+### Concurrency and State
+- Use `kotlinx.atomicfu` for thread-safe operations across platforms.
+- Prefer coroutines over callbacks where possible.
+- Transport implementations extend `AbstractTransport` for consistent callback management.
+
+### Error Handling
+- Use sealed classes for representing different result states.
+- Map errors to JSON-RPC error codes in protocol handlers.
+- Log errors using `io.github.oshai.kotlinlogging.KotlinLogging`.
+
+### Logging
+- Use `KotlinLogging.logger {}` for structured logging.
+- Never log sensitive data (credentials, tokens).
+
+## Pull Requests
+- Base all PRs on the `main` branch.
+- PR title format: Brief description of the change
+- Include in PR description:
+  - **What changed?**
+  - **Why? Motivation and context**
+  - **Breaking changes?** (if any)
+- Run `./gradlew apiCheck` before submitting.
+- Link PR to related issue (except for minor docs/typo fixes).
+- Follow [Kotlin Coding Conventions](https://kotlinlang.org/docs/reference/coding-conventions.html).
+
+## Review Checklist
+- `./gradlew apiCheck` must pass.
+- `./gradlew test` must succeed for affected modules.
+- New tests added for any new feature or bug fix.
+- Documentation updated for user-facing changes.
+- No breaking changes to public APIs without discussion.