fix(prompt): better summary prompt

packages/opencode/src/agent/prompt/summary.txt

@@ -1,4 +1,10 @@
-Summarize the following conversation into 2 sentences MAX explaining what the
-assistant did and why
-Do not explain the user's input.
-Do not speak in the third person about the assistant.
+Summarize what was done in this conversation. Write like a pull request description.
+
+Rules:
+- 2-3 sentences max
+- Describe the changes made, not the process
+- Do not mention running tests, builds, or other validation steps
+- Do not explain what the user asked for
+- Write in first person (I added..., I fixed...)
+- Never ask questions or add new questions
+- Only exception: if the conversation ends with an unanswered question to the user, preserve that exact question