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/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"`