fix(acp): parse session/update content as ContentBlock, not string

The ACP spec defines session/update content as a ContentBlock object
({type, text}) but we decoded it as a string. This dropped every
agent_message_chunk and left prompt responses empty. E2E against the
real opencode acp binary surfaced the bug.

- internal/acp/messages.go: add ContentBlock struct; Update.Content is
  now ContentBlock.
- internal/acp/client.go: handleNotification extracts Text when
  Content.Type == "text"; other types are logged at debug and dropped.
- internal/acp/client_test.go: NDJSON framing test sends content as a
  ContentBlock object and reads .Text.
- README.md: document /acp/* routes + CODESPACE_OPENCODE_ARGS env.

Verified end-to-end: POST /acp/prompt returns text="Hi there!", 4
messages in history (1 user + 3 agent chunks), 0 invalid session/update
log entries.

Conversation: 019f357d-4236-7010-949c-e61067618d42

internal/acp/client.go

@@ -237,9 +237,17 @@ func (c *Client) handleNotification(method string, params json.RawMessage) {
 	}
 	switch up.Update.SessionUpdate {
 	case "agent_message_chunk":
-		c.history.Add("agent", up.Update.Content)
+		if up.Update.Content.Type == "text" {
+			c.history.Add("agent", up.Update.Content.Text)
+		} else {
+			c.lg.Debug("acp drop non-text agent chunk", "workspace_id", c.workspaceID, "type", up.Update.Content.Type)
+		}
 	case "user_message_chunk":
-		c.history.Add("user", up.Update.Content)
+		if up.Update.Content.Type == "text" {
+			c.history.Add("user", up.Update.Content.Text)
+		} else {
+			c.lg.Debug("acp drop non-text user chunk", "workspace_id", c.workspaceID, "type", up.Update.Content.Type)
+		}
 	default:
 		c.lg.Debug("acp drop update", "workspace_id", c.workspaceID, "type", up.Update.SessionUpdate)
 	}

internal/acp/client_test.go

@@ -115,11 +115,11 @@ func TestNDJSONFramingHandlesSplitLines(t *testing.T) {
 		got <- struct {
 			method  string
 			content string
-		}{method, up.Update.Content}
+		}{method, up.Update.Content.Text}
 	}
 
-	line1 := `{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"s1","update":{"sessionUpdate":"agent_message_chunk","content":"hello"}}}`
-	line2 := `{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"s1","update":{"sessionUpdate":"agent_message_chunk","content":" world"}}}`
+	line1 := `{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"s1","update":{"sessionUpdate":"agent_message_chunk","content":{"type":"text","text":"hello"}}}}`
+	line2 := `{"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"s1","update":{"sessionUpdate":"agent_message_chunk","content":{"type":"text","text":" world"}}}}`
 
 	// Send one complete line plus a partial second line.
 	sub.send([]byte(line1 + "\n" + line2[:len(line2)-2]))

internal/acp/messages.go

@@ -93,10 +93,16 @@ type SessionUpdateParams struct {
 type Update struct {
 	SessionUpdate string `json:"sessionUpdate"`
 	MessageID     string `json:"messageId,omitempty"`
-	Content       string `json:"content,omitempty"`
+	Content       ContentBlock `json:"content,omitempty"`
 	ToolCallID    string `json:"toolCallId,omitempty"`
 }
 
+// ContentBlock is a content payload in session/update notifications.
+type ContentBlock struct {
+	Type string `json:"type"`
+	Text string `json:"text,omitempty"`
+}
+
 // FSReadParams is sent by the agent to read a workspace file.
 type FSReadParams struct {
 	Path      string `json:"path"`