Add PromptRequest.text() convenience method, fix snapshot versions

Add text() to PromptRequest that returns the first TextContent string
from the prompt list, replacing the 5-line stream/filter/map/findFirst
boilerplate. README installation section now correctly references
0.9.0-SNAPSHOT with the snapshot repository configuration.

Author: mark-tuvium

README.md

@@ -26,13 +26,22 @@ Three API styles for building agents:
 
 ## Installation
 
-Published to Maven Central:
+Snapshot builds are published to Maven Central Snapshots. Add the snapshot repository and dependency:
 
 ```xml
+<repositories>
+    <repository>
+        <id>central-snapshots</id>
+        <url>https://central.sonatype.com/repository/maven-snapshots/</url>
+        <snapshots><enabled>true</enabled></snapshots>
+        <releases><enabled>false</enabled></releases>
+    </repository>
+</repositories>
+
 <dependency>
     <groupId>com.agentclientprotocol</groupId>
     <artifactId>acp-core</artifactId>
-    <version>0.9.0</version>
+    <version>0.9.0-SNAPSHOT</version>
 </dependency>
 ```
 
@@ -41,7 +50,7 @@ For annotation-based agent development:
 <dependency>
     <groupId>com.agentclientprotocol</groupId>
     <artifactId>acp-agent-support</artifactId>
-    <version>0.9.0</version>
+    <version>0.9.0-SNAPSHOT</version>
 </dependency>
 ```
 
@@ -50,7 +59,7 @@ For WebSocket server support (agents accepting WebSocket connections):
 <dependency>
     <groupId>com.agentclientprotocol</groupId>
     <artifactId>acp-websocket-jetty</artifactId>
-    <version>0.9.0</version>
+    <version>0.9.0-SNAPSHOT</version>
 </dependency>
 ```
 
@@ -451,10 +460,10 @@ This SDK is part of the [Agent Client Protocol](https://agentclientprotocol.com/
 - Capability negotiation
 - Structured error handling
 - Full protocol compliance (all SessionUpdate types, MCP configs, `_meta` extensibility)
-- Snapshot builds published to Maven Central
+- Snapshot builds published to Maven Central Snapshots
 - 258 tests
 
 ### v1.0.0 (Planned)
-- Stable release to Maven Central
+- Stable release published to Maven Central
 - Production hardening
 - Performance optimizations

acp-core/src/main/java/com/agentclientprotocol/sdk/spec/AcpSchema.java

@@ -312,6 +312,21 @@ public record PromptRequest(@JsonProperty("sessionId") String sessionId,
 		public PromptRequest(String sessionId, List<ContentBlock> prompt) {
 			this(sessionId, prompt, null);
 		}
+
+		/**
+		 * Returns the text of the first {@link TextContent} block in the prompt, or an empty
+		 * string if no text content is present.
+		 */
+		public String text() {
+			if (prompt == null) {
+				return "";
+			}
+			return prompt.stream()
+				.filter(c -> c instanceof TextContent)
+				.map(c -> ((TextContent) c).text())
+				.findFirst()
+				.orElse("");
+		}
 	}
 
 	/**