Add READMEs for acp-core, acp-annotations, acp-test, acp-websocket-jetty

Lightweight module READMEs with key classes, installation, and doc links.

Author: mark-tuvium

acp-annotations/README.md

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+# acp-annotations
+
+Zero-dependency annotation library for declarative ACP agent development. Use with `acp-agent-support` for annotation processing at runtime.
+
+## Annotations
+
+| Annotation | Target | Purpose |
+|-----------|--------|---------|
+| `@AcpAgent` | Class | Marks a class as an ACP agent |
+| `@Initialize` | Method | Handles initialization requests |
+| `@NewSession` | Method | Handles new session creation |
+| `@LoadSession` | Method | Handles session resume |
+| `@Prompt` | Method | Handles prompt requests |
+| `@Cancel` | Method | Handles cancel notifications |
+| `@SessionState` | Parameter | Injects session-scoped state |
+| `@SetSessionMode` | Method | Handles mode change requests |
+| `@SetSessionModel` | Method | Handles model change requests |
+
+## Installation
+
+```xml
+<dependency>
+    <groupId>com.agentclientprotocol</groupId>
+    <artifactId>acp-annotations</artifactId>
+    <version>0.9.0</version>
+</dependency>
+```
+
+## Documentation
+
+- [Annotation-based Agent API](https://springaicommunity.mintlify.app/acp-java-sdk/reference/java#annotation-based-api)
+- [Tutorial](https://springaicommunity.mintlify.app/acp-java-sdk/tutorial)

acp-core/README.md

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+# acp-core
+
+Core implementation of the Agent Client Protocol for Java. Provides client and agent APIs, protocol types, stdio transport, and Project Reactor-based async support.
+
+## Key Classes
+
+| Class | Purpose |
+|-------|---------|
+| `AcpClient` | Factory — `AcpClient.sync()` and `AcpClient.async()` |
+| `AcpAgent` | Factory — `AcpAgent.sync()` and `AcpAgent.async()` |
+| `AcpSchema` | All protocol types (requests, responses, updates) |
+| `StdioAcpClientTransport` | Launches agent as subprocess, communicates over stdin/stdout |
+| `StdioAcpAgentTransport` | Reads from stdin, writes to stdout |
+
+## Installation
+
+```xml
+<dependency>
+    <groupId>com.agentclientprotocol</groupId>
+    <artifactId>acp-core</artifactId>
+    <version>0.9.0</version>
+</dependency>
+```
+
+## Documentation
+
+- [API Reference](https://springaicommunity.mintlify.app/acp-java-sdk/reference/java)
+- [Tutorial](https://springaicommunity.mintlify.app/acp-java-sdk/tutorial)

acp-test/README.md

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+# acp-test
+
+Test utilities for unit testing ACP clients and agents without subprocesses or network connections.
+
+## Key Classes
+
+| Class | Purpose |
+|-------|---------|
+| `InMemoryTransportPair` | Creates connected client/agent transports in-process |
+| `MockAcpAgent` | Records requests, provides configurable responses |
+| `MockAcpClient` | Records session updates, handles agent requests |
+
+## Installation
+
+```xml
+<dependency>
+    <groupId>com.agentclientprotocol</groupId>
+    <artifactId>acp-test</artifactId>
+    <version>0.9.0</version>
+    <scope>test</scope>
+</dependency>
+```
+
+## Usage
+
+```java
+var pair = InMemoryTransportPair.create();
+
+// Build client with one side
+AcpSyncClient client = AcpClient.sync(pair.clientTransport()).build();
+
+// Build agent with the other
+AcpSyncAgent agent = AcpAgent.sync(pair.agentTransport())
+    .initializeHandler(req -> InitializeResponse.ok())
+    .promptHandler((req, ctx) -> { /* ... */ })
+    .build();
+```
+
+## Documentation
+
+- [Module 16: In-Memory Testing](https://springaicommunity.mintlify.app/acp-java-sdk/tutorial/16-in-memory-testing)

acp-websocket-jetty/README.md

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+# acp-websocket-jetty
+
+WebSocket transport for ACP agents using embedded Jetty. Allows agents to accept WebSocket connections from clients instead of using stdio.
+
+## Key Classes
+
+| Class | Purpose |
+|-------|---------|
+| `WebSocketAcpAgentTransport` | Embedded Jetty WebSocket server for agents |
+
+## Installation
+
+```xml
+<dependency>
+    <groupId>com.agentclientprotocol</groupId>
+    <artifactId>acp-websocket-jetty</artifactId>
+    <version>0.9.0</version>
+</dependency>
+```
+
+## When to Use
+
+Use WebSocket transport when the agent runs as a long-lived server process that multiple clients connect to over the network. For single-client CLI usage, stdio transport (`acp-core`) is simpler.
+
+## Documentation
+
+- [API Reference — Transports](https://springaicommunity.mintlify.app/acp-java-sdk/reference/java#transports)