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/tools/call-omo-agent/sync-executor.test.ts

@@ -394,6 +394,8 @@ describe("executeSync", () => {
       expect(observed[0]?.bootstrap?.tools?.question).toBe(false)
       expect(observed[0]?.bootstrap?.fallbackChain?.[0]?.model).toBe("gpt-5.4")
       expect(getDelegatedChildSessionBootstrap("ses-call-bootstrap")).toBeUndefined()
+      // session-agent state for a sync session we created must be cleared after dispatch
+      expect(getSessionAgent("ses-call-bootstrap")).toBeUndefined()
     } finally {
       clearAllDelegatedChildSessionBootstrap()
       clearSessionTools()

src/tools/call-omo-agent/sync-executor.ts

@@ -1,5 +1,5 @@
 import type { PluginInput } from "@opencode-ai/plugin"
-import { setSessionAgent, subagentSessions, syncSubagentSessions } from "../../features/claude-code-session-state"
+import { clearSessionAgent, setSessionAgent, subagentSessions, syncSubagentSessions } from "../../features/claude-code-session-state"
 import { promptAsyncAfterSessionIdle } from "../../hooks/shared/prompt-async-gate"
 import { getAgentToolRestrictions, log } from "../../shared"
 import { getAgentDisplayName, stripAgentListSortPrefix } from "../../shared/agent-display-names"
@@ -187,6 +187,7 @@ export async function executeSync(
       subagentSessions.delete(sessionID)
       syncSubagentSessions.delete(sessionID)
       deleteSessionTools(sessionID)
+      clearSessionAgent(sessionID)
     }
   }
 }