fix(session): preserve custom agent after switching (#1017)

Use setSessionAgent (first-write wins) instead of updateSessionAgent in chat.message handler. This prevents the default agent from overwriting a custom agent that was set via UI switch.

Fixes #893

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: justsisyphus <justsisyphus@users.noreply.github.com>
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/index.ts

@@ -314,7 +314,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
 
     "chat.message": async (input, output) => {
       if (input.agent) {
-        updateSessionAgent(input.sessionID, input.agent);
+        setSessionAgent(input.sessionID, input.agent);
       }
 
       const message = (output as { message: { variant?: string } }).message