fix: orphan sub-sessions not cleaned up when parent aborts #37314

Description

When a parent session is aborted (e.g., due to timeout, user cancellation, or runtime error), its sub-agent sessions are not automatically cleaned up. The orphaned sub-sessions remain in tool-calls state, consuming resources indefinitely.

Steps to reproduce

1. Start an agent session that dispatches sub-agents via task(subagent_type="...")
2. Allow one or more sub-agents to get stuck (e.g., infinite explore loop)
3. Abort the parent session
4. Observe that child sub-agent sessions remain in tool-calls state indefinitely

Evidence

From a production opencode deployment:

• 4 orphan sub-sessions currently stuck in tool-calls state with terminated parents:

  • ses_09e82cb4 (explore sub-agent) — parent task-executor completed days ago
  • ses_0c77d7ac (explore sub-agent) — parent fusion-deep-research completed
  • ses_0ddbf20a (critical-thinking sub-agent) — parent fusion-deep-research completed
  • ses_117c5c57 (critical-thinking sub-agent) — parent session completed

• 5 mass-abort events where 3+ sub-agents were aborted together because their parent was aborted — but the cleanup was a batch abort, not a graceful shutdown:

  • ses_10049ce8: 4 children aborted together (3 pm-coordinator + 1 explore)
  • ses_0c781f98: 3 children aborted together (pm-coordinator + dify-kb-manager + communication-drafter)
  • ses_0d4228cd: 3 children aborted together
  • ses_0dab015c2: 3 explore children aborted together
  • ses_11846e3e: 3 children aborted together

Expected behavior

1. Cascading abort: When a parent session is terminated, automatically abort all active child sub-sessions.
2. Orphan detection: Periodic cleanup of sub-sessions whose parent session no longer exists or has been terminated.
3. Graceful shutdown signal: When aborting, send a cancellation signal to sub-agents first, wait briefly for them to save state, then force-terminate.

Impact

• Prevents resource leaks from orphaned sub-sessions
• Avoids confusion when checking session status and finding "running" sessions with dead parents