fix(claude-code-session-state): clear session-agent map on delete and sync cleanup

External review on PR #4074 noted that adding setSessionAgent for child
sessions left sessionAgentMap holding entries after the session was
deleted or after the sync call_omo_agent executor cleaned up other
owned state. The map only grows; entries never get reused but they do
accumulate across long-running plugin instances.

Close both gaps:
- BackgroundManager.handleEvent for session.deleted now calls
  clearSessionAgent for the deleted session id on both the early-return
  no-task branch and the cascade tail. This pairs with the existing
  clearDelegatedChildSessionBootstrap and SessionCategoryRegistry.remove
  so all owned session state is dropped together.
- sync-executor finally for createdSessionForExecution now calls
  clearSessionAgent alongside the existing subagentSessions,
  syncSubagentSessions, and deleteSessionTools cleanup so sessions this
  executor created cannot leak their agent mapping.

Adds focused tests:
- BackgroundManager.handleEvent - session.deleted cascade > should
  clear session agent state for deleted sessions to prevent map leak
- executeSync > registers child-session bootstrap and tracked prompt
  state before sync prompt dispatch (extended assertion for cleanup)

src/features/background-agent/manager.ts

@@ -1660,6 +1660,7 @@ The fallback retry session is now created and can be inspected directly.
 
       if (tasksToCancel.size === 0) {
         this.clearTaskHistoryWhenParentTasksGone(sessionID)
+        clearSessionAgent(sessionID)
         return
       }
 
@@ -1698,6 +1699,7 @@ The fallback retry session is now created and can be inspected directly.
 
       this.rootDescendantCounts.delete(sessionID)
       clearDelegatedChildSessionBootstrap(sessionID)
+      clearSessionAgent(sessionID)
       SessionCategoryRegistry.remove(sessionID)
     }