feat: add structured stderr output for error diagnostics (#29)

Add StderrOutput schema with head/tail/truncated/total_lines fields to
provide better error diagnostics when agent processes fail.

server/packages/universal-agent-schema/src/agents/amp.rs

@@ -109,6 +109,9 @@ pub fn event_to_universal(event: &schema::StreamJsonMessage) -> Result<Vec<Event
                     UniversalEventData::SessionEnded(SessionEndedData {
                         reason: SessionEndReason::Completed,
                         terminated_by: TerminatedBy::Agent,
+                        message: None,
+                        exit_code: None,
+                        stderr: None,
                     }),
                 )
                 .with_raw(serde_json::to_value(event).ok()),

server/packages/universal-agent-schema/src/agents/codex.rs

@@ -521,6 +521,9 @@ pub fn session_ended_event(thread_id: &str, reason: SessionEndReason) -> EventCo
         UniversalEventData::SessionEnded(SessionEndedData {
             reason,
             terminated_by: TerminatedBy::Agent,
+            message: None,
+            exit_code: None,
+            stderr: None,
         }),
     )
     .with_native_session(Some(thread_id.to_string()))

server/packages/universal-agent-schema/src/lib.rs

@@ -79,9 +79,33 @@ pub struct SessionStartedData {
 pub struct SessionEndedData {
     pub reason: SessionEndReason,
     pub terminated_by: TerminatedBy,
+    /// Error message when reason is Error
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub message: Option<String>,
+    /// Process exit code when reason is Error
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub exit_code: Option<i32>,
+    /// Agent stderr output when reason is Error
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub stderr: Option<StderrOutput>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, JsonSchema, ToSchema)]
+pub struct StderrOutput {
+    /// First N lines of stderr (if truncated) or full stderr (if not truncated)
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub head: Option<String>,
+    /// Last N lines of stderr (only present if truncated)
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub tail: Option<String>,
+    /// Whether the output was truncated
+    pub truncated: bool,
+    /// Total number of lines in stderr
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub total_lines: Option<usize>,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize, JsonSchema, ToSchema)]
 #[serde(rename_all = "snake_case")]
 pub enum SessionEndReason {
     Completed,