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/opencode/src/cli/cmd/github.ts

@@ -641,7 +641,7 @@ export const GithubRunCommand = cmd({
           messageID: Identifier.ascending("message"),
           providerID,
           modelID,
-          mode: "build",
+          agent: "build",
           parts: [
             {
               id: Identifier.ascending("part"),