fix(agents): preserve original prompt on model fallback retry (#65760)

When a model call fails and OpenClaw triggers a fallback retry,
resolveFallbackRetryPrompt() replaced the entire original user body with
the fixed string "Continue where you left off. The previous model
attempt failed or timed out." This discarded the original task
instruction entirely, forcing the fallback model to reconstruct the task
from history alone — sometimes impossible, sometimes just wrong.

Prepend the retry context to the original body instead so the fallback
model has both the recovery signal and the task instruction:

    [Retry after the previous model attempt failed or timed out]

    <original body>

The existing subagent-spawn branch (no persisted session history) is
unaffected — that path still returns params.body verbatim.

Update the matching test to assert the new prefixed form.

Closes #65760

src/agents/command/attempt-execution.helpers.ts

@@ -73,7 +73,13 @@ export function resolveFallbackRetryPrompt(params: {
   if (!params.sessionHasHistory) {
     return params.body;
   }
-  return "Continue where you left off. The previous model attempt failed or timed out.";
+  // Even with persisted session history, fully replacing the body with a
+  // generic "continue where you left off" message strips the original task
+  // from the fallback model's view. Agents then have to reconstruct the
+  // instruction from history alone, which is fragile and sometimes
+  // impossible. Prepend the retry context to the original body instead so
+  // the fallback model has both the recovery signal AND the task. (#65760)
+  return `[Retry after the previous model attempt failed or timed out]\n\n${params.body}`;
 }
 
 export function createAcpVisibleTextAccumulator() {

src/agents/command/attempt-execution.test.ts

@@ -20,14 +20,14 @@ describe("resolveFallbackRetryPrompt", () => {
     ).toBe(originalBody);
   });
 
-  it("returns recovery prompt for fallback retry with existing session history", () => {
+  it("prepends recovery prefix to original body on fallback retry with existing session history", () => {
     expect(
       resolveFallbackRetryPrompt({
         body: originalBody,
         isFallbackRetry: true,
         sessionHasHistory: true,
       }),
-    ).toBe("Continue where you left off. The previous model attempt failed or timed out.");
+    ).toBe(`[Retry after the previous model attempt failed or timed out]\n\n${originalBody}`);
   });
 
   it("preserves original body for fallback retry when session has no history (subagent spawn)", () => {