Update README with tool execution model

Document the three tool types: activity stubs (auto-detected),
@SideEffectTool (wrapped in sideEffect), and plain tools (execute
in workflow context, user responsible for determinism).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: donald-pinckney

temporal-spring-ai/README.md

@@ -31,9 +31,12 @@ The plugin auto-registers `ChatModelActivity` with all Temporal workers. In your
 @WorkflowInit
 public MyWorkflowImpl(String goal) {
     ActivityChatModel chatModel = ActivityChatModel.forDefault();
+
+    WeatherActivity weather = Workflow.newActivityStub(WeatherActivity.class, opts);
+
     this.chatClient = TemporalChatClient.builder(chatModel)
             .defaultSystem("You are a helpful assistant.")
-            .defaultTools(myActivityStub)
+            .defaultTools(weather, new MyTools())
             .build();
 }
 
@@ -43,6 +46,52 @@ public String run(String goal) {
 }
 ```
 
+## Tool Types
+
+Tools passed to `defaultTools()` are handled based on their type:
+
+### Activity stubs
+
+Interfaces annotated with both `@ActivityInterface` and `@Tool` methods. Auto-detected and executed as durable Temporal activities with retries and timeouts.
+
+```java
+@ActivityInterface
+public interface WeatherActivity {
+    @Tool(description = "Get weather for a city") @ActivityMethod
+    String getWeather(String city);
+}
+```
+
+### `@SideEffectTool`
+
+Classes annotated with `@SideEffectTool`. Each `@Tool` method is wrapped in `Workflow.sideEffect()` — the result is recorded in history on first execution and replayed from history on subsequent replays. Use for cheap non-deterministic operations (timestamps, UUIDs).
+
+```java
+@SideEffectTool
+public class TimestampTools {
+    @Tool(description = "Get current time")
+    public String now() { return Instant.now().toString(); }
+}
+```
+
+### Plain tools
+
+Any class with `@Tool` methods that isn't a stub or `@SideEffectTool`. Executes directly in the workflow thread. The user is responsible for determinism — call activities, `Workflow.sideEffect()`, child workflows, etc. as needed.
+
+```java
+public class MyTools {
+    @Tool(description = "Process data")
+    public String process(String input) {
+        SomeActivity act = Workflow.newActivityStub(SomeActivity.class, opts);
+        return act.doWork(input);
+    }
+}
+```
+
+### Nexus service stubs
+
+Auto-detected and executed as Nexus operations, similar to activity stubs.
+
 ## Optional Integrations
 
 These are auto-configured when their dependencies are on the classpath: