Merge agent and mode into one (#1689)

The concept of mode has been deprecated, there is now only the agent field in the config.

An agent can be cycled through as your primary agent with <tab> or you can spawn a subagent by @ mentioning it. if you include a description of when to use it, the primary agent will try to automatically use it

Full docs here: https://opencode.ai/docs/agents/

packages/sdk/go/session_test.go

@@ -129,8 +129,8 @@ func TestSessionChatWithOptionalParams(t *testing.T) {
 				}),
 			}}),
 			ProviderID: opencode.F("providerID"),
+			Agent:      opencode.F("agent"),
 			MessageID:  opencode.F("msg"),
-			Mode:       opencode.F("mode"),
 			System:     opencode.F("system"),
 			Tools: opencode.F(map[string]bool{
 				"foo": true,