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.test.ts

@@ -5063,6 +5063,27 @@ describe("BackgroundManager.handleEvent - session.deleted cascade", () => {
 
     manager.shutdown()
   })
+
+  test("should clear session agent state for deleted sessions to prevent map leak", async () => {
+    //#given
+    const { setSessionAgent } = await import("../claude-code-session-state")
+    resetClaudeCodeSessionState()
+    const manager = createBackgroundManager()
+    const sessionID = "session-deleted-agent-leak"
+    setSessionAgent(sessionID, "sisyphus-junior")
+    expect(getSessionAgent(sessionID)).toBe("sisyphus-junior")
+
+    //#when
+    manager.handleEvent({
+      type: "session.deleted",
+      properties: { info: { id: sessionID } },
+    })
+
+    //#then
+    expect(getSessionAgent(sessionID)).toBeUndefined()
+
+    manager.shutdown()
+  })
 })
 
 describe("BackgroundManager.handleEvent - session.error", () => {

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)
     }