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)
 	}