tweak: wrap build-switch w/ system-reminder (#2525)

packages/opencode/src/session/index.ts

@@ -766,8 +766,8 @@ export namespace Session {
       })
     }
 
-    const lastAssistantMsg = msgs.filter((x) => x.info.role === "assistant").at(-1)?.info as MessageV2.Assistant
-    if (lastAssistantMsg?.mode === "plan" && agent.name === "build") {
+    const wasPlan = msgs.some((msg) => msg.info.role === "assistant" && msg.info.mode === "plan")
+    if (wasPlan && agent.name === "build") {
       msgs.at(-1)?.parts.push({
         id: Identifier.ascending("part"),
         messageID: userMsg.id,

packages/opencode/src/session/prompt/build-switch.txt

@@ -1 +1,5 @@
-Your operational mode has changed from plan to build. You are no longer in read-only mode. You are permitted to make file changes as necessary and utilize your arsenal of tools as needed.
+<system-reminder>
+Your operational mode has changed from plan to build.
+You are no longer in read-only mode.
+You are permitted to make file changes, run shell commands, and utilize your arsenal of tools as needed.
+</system-reminder>