fix(agents): mark sensitive-stop payload as error with isError flag

Add isError: true to the sensitive stop_reason payload so downstream consumers (e.g. cron delivery) correctly treat it as an error. Add 'sensitive' to the error kind union type.

src/agents/pi-embedded-runner/run.ts

@@ -1320,6 +1320,7 @@ export async function runEmbeddedPiAgent(
                     text:
                       "I can't respond to that - the content was flagged by the model's safety filter. " +
                       "Please rephrase or try a different topic.",
+                    isError: true,
                   },
                 ],
                 meta: {

src/agents/pi-embedded-runner/types.ts

@@ -41,7 +41,8 @@ export type EmbeddedPiRunMeta = {
       | "compaction_failure"
       | "role_ordering"
       | "image_size"
-      | "retry_limit";
+      | "retry_limit"
+      | "sensitive";
     message: string;
   };
   /** Stop reason for the agent run (e.g., "completed", "tool_calls"). */