Implement metadata merging in context for sub-agent hooks

- Add a new `MergeMetadata` function in the `Context` struct to allow merging of caller-provided metadata into the context, enabling sub-agent hooks to access this information.
- Update the `Stream` method in the `Assistant` to utilize the new metadata merging functionality, enhancing the context management for sub-agents.

agent/assistant/agent.go

@@ -55,6 +55,9 @@ func (ast *Assistant) Stream(ctx *context.Context, inputMessages []context.Messa
 		opts = &context.Options{}
 	}
 
+	// Merge caller-provided metadata into ctx so sub-agent hooks can read it via ctx.metadata
+	ctx.MergeMetadata(opts.Metadata)
+
 	// Initialize stack and auto-handle completion/failure/restore
 	_, _, done := context.EnterStack(ctx, ast.ID, opts)
 	defer done()

agent/context/context.go

@@ -541,3 +541,19 @@ func (ctx *Context) IsA2ACall() bool {
 func (ctx *Context) IsForkedA2ACall() bool {
 	return ctx.Referer == RefererAgentFork
 }
+
+// MergeMetadata merges the given metadata into ctx.Metadata.
+// Existing keys are overwritten by incoming values.
+// This enables A2A callers to pass custom metadata (e.g. oneshot, async)
+// to sub-agent hooks via ctx.metadata in JavaScript.
+func (ctx *Context) MergeMetadata(metadata map[string]interface{}) {
+	if len(metadata) == 0 {
+		return
+	}
+	if ctx.Metadata == nil {
+		ctx.Metadata = make(map[string]interface{}, len(metadata))
+	}
+	for k, v := range metadata {
+		ctx.Metadata[k] = v
+	}
+}