docs(coding-agent): guide completed subagent cleanup

[sethkarten]

Summary

• tell the model to retain completed direct children when they may be reused
• tell the model to delete a completed direct child only once it is definitively no longer needed
• keep lifecycle cleanup explicit rather than automatically deleting reusable sessions

Tests

• test/system-prompt.test.ts
• npm run check

Stacked on #547.

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Note

Low Risk
Prompt and changelog-only change; no runtime or API behavior is modified.

Overview
Subagent lifecycle guidance in buildSubagentGuidance now tells the model to keep finished direct children when they might be reused (e.g. via agent_message), and to call await rlm.delete_subagent(child) only after they are clearly no longer needed—making cleanup explicit instead of implying automatic removal of reusable sessions.

The [Unreleased] changelog entry and a system-prompt test assert the new sentence is present in the assembled prompt.

^{Reviewed by Cursor Bugbot for commit b8365ac. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Retry deleted subagent runtime cleanup after compaction and guide subagent lifecycle

• Adds _reapDeletedRlmSubagentRuntimesAfterCompaction to AgentSession, which retries deleteRlmSubagent for any pending retryable deletions using Promise.allSettled after each compaction.
• Updates buildSubagentGuidance to advise retaining reusable completed direct children and explicitly deleting them with rlm.delete_subagent when no longer needed.
• Adds and updates tests in agent-session-recursion.test.ts and agent-session-compaction.test.ts to cover the new reap-on-compact behavior, including error tolerance.

^{Macroscope summarized b8365ac.}

[snimu]

I'd prefer to release all subagents that aren't currently running on every compaction, so it's only on disk. And then when the main agent wants to use them again, rehydrate that specific subagent.
That would ease the load on the agent and reduce distractions, increase reliability, enable re-using older agents again, and likely save more memory.