fix: Fix session race condition killing active agents and retry loop (#1627)

## Problem

Fix session race condition killing active agents and retry loop.

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<!-- Closes #ISSUE_ID -->

## Changes

1. Scope process cleanup to the current taskRunId instead of killing all agent/child processes for the task
2. Clear initialPrompt on session error to prevent retry loops

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## How did you test this?

Manually

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Author: charlesvien

apps/code/src/main/services/agent/service.ts

@@ -573,7 +573,10 @@ When creating pull requests, add the following footer at the end of the PR descr
       }
 
       for (const proc of this.processTracking.getByTaskId(taskId)) {
-        if (proc.category === "agent" || proc.category === "child") {
+        if (
+          (proc.category === "agent" || proc.category === "child") &&
+          proc.metadata?.taskRunId === taskRunId
+        ) {
           this.processTracking.kill(proc.pid);
         }
       }

apps/code/src/renderer/features/sessions/service/service.ts

@@ -1121,6 +1121,7 @@ export class SessionService {
           isPromptPending: false,
           isCompacting: false,
           promptStartedAt: null,
+          initialPrompt: undefined,
         });
       } else {
         sessionStoreSetters.updateSession(session.taskRunId, {