fix(agent): preserve thrown error values

docs/core-data-structures/session.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write
-session.md: f439b3cb681a4073bba76cfeb23e8711000da797
-session.zh.md: 8da3384900a1963de61552e3e72af8bbb1f509d6
+session.md: 6401a1dd4111ab157138e8fed53754c41db7022c
+session.zh.md: e3573c2836b598bcad11835de80b10c59386917c

docs/core-data-structures/session.md

@@ -516,7 +516,8 @@ interface TurnEndReasonMap {
    * step number the failure occurred on (the operational error's location — the
    * single durable record of an in-turn failure; live diagnostics also fire via
    * `agent/error`). Final model-request failures retain their normalized facts
-   * as one `failure`; other turn failures retain their live Error message/code.
+   * as one `failure`; other thrown values retain their rendered message and a
+   * real `HarnessError` code when present.
    */
   error: { kind: 'error'; step: number } & (
     | { failure: LlmFailure; message?: never; code?: never }

docs/core-data-structures/session.zh.md

@@ -516,7 +516,8 @@ interface TurnEndReasonMap {
    * step number the failure occurred on (the operational error's location — the
    * single durable record of an in-turn failure; live diagnostics also fire via
    * `agent/error`). Final model-request failures retain their normalized facts
-   * as one `failure`; other turn failures retain their live Error message/code.
+   * as one `failure`; other thrown values retain their rendered message and a
+   * real `HarnessError` code when present.
    */
   error: { kind: 'error'; step: number } & (
     | { failure: LlmFailure; message?: never; code?: never }