refactor(coding-agent): wake RLM quiescence on activity changes instead of polling

[snimu]

Note: this PR also contains the full change from #1844 (serialized post-compaction continuation: one settlement owner, commit-fence re-checks, and paused-continuation preservation replacing the racy fixed delay) - that PR is closed and merges here as one unit.

What was wrong

waitForRlmQuiescence — the strong idle barrier gating goal continuations and daemon idle for sessions with subagent work — detected settlement by re-checking on a zero-delay setTimeout(0) loop: busy-spinning the event loop instead of being told when activity changes (audit: timeouts.md finding 7).

The fix

The poll is replaced by an abort-aware activity-change waiter that reuses the existing session-input checkpoint waiter set (no second notification registry). Every component of the quiescence predicate now notifies on its clear path — bash, refine, retry, compaction, branch summaries, terminal actions, and (added in review) the post-compaction continuation settlement, which was the one clear path without a notify and could have hung the barrier permanently in a narrow registration window. Spurious wakes are safe in both directions: all consumers of the shared set re-check their predicates in loops. +31/−9.

How it's verified

Reviewer performed an exhaustive missed-wake audit of every isSessionActive component with per-clear-site accounting, verified bidirectional spurious-wake safety across the shared waiter set, confirmed no hot-path notifies (all fire once per operation, never per-token), and validated the goal-continuation-quiescence suite (#1610) is untouched and green (7/7). Recursion suite 116/116; full CI-style failing set matches stack base. Two-model implement/review loop, approved on second pass.

Stacked on #1751 (test the whole stack at the leaf; merge base-first).

Note: intentionally no Linear ticket for this cleanup stack, so that check stays red.

---

Note

Medium Risk
Changes core session scheduling, compaction continuation, and RLM idle barriers; regressions could stall continuations, leak admission waiters, or mis-order resume vs queued input, though behavior is heavily covered by updated compaction, queue, and recursion tests.

Overview
Replaces busy-wait patterns in RLM quiescence and post-compaction continuation with waits on shared session activity notifications.

waitForRlmQuiescence no longer spins on setTimeout(0) when session-level work is active; it blocks on a new _waitForSessionActivityChange hook that shares the existing checkpoint waiter set. _notifySessionInputCheckpointChange is invoked when that work finishes (bash, refine apply, retry, compaction, branch summary, post-compaction settlement, and related paths) so waiters wake instead of polling.

Post-compaction resume drops the fixed 100ms timer. _runScheduledPostCompactionContinue runs immediately and loops until it can safely call agent.continue(), explicitly waiting for agent idle, retries, refine, an in-flight _compactionOperation, queued-work pauses, and session-input pump idle. A continueAfterSessionInput flag on the continuation settlement controls whether an empty “resume after compaction” should still call continue() once queued session input has drained.

waitForIdle / checkpoint waiting now register and clean up waiters correctly and can Promise.race against a superseded post-compaction settlement so cancelled continuations do not leak waiters and block daemon passivation.

^{Reviewed by Cursor Bugbot for commit 5bcedd4. Bugbot is set up for automated code reviews on this repo. Configure here.}

Linear ticket: ENG-5667
(ticket linked above)

Note

Replace polling with event-driven waiters for RLM quiescence and post-compaction continuations

• Introduces AgentSession._waitForSessionActivityChange as a one-shot waiter that resolves on session activity or abort, replacing setTimeout(0) busy-loop yielding in waitForRlmQuiescence
• Removes fixed-delay timer scheduling from post-compaction continuations; _schedulePostCompactionContinue now launches _runScheduledPostCompactionContinue directly, which loops waiting for idle, retry, refine, queued-work, and compaction completion
• Adds continueAfterSessionInput flag to PostCompactionContinuationSettlement so continuations can optionally wait for pending session input before calling agent.continue()
• Notifies session-input checkpoint waiters from runUserBash, refine-application paths, branch summary, retry resolution, and compaction completion so quiescence waiters wake promptly
• Risk: waitForRlmQuiescence no longer polls on a macrotask interval; any missed _notifySessionInputCheckpointChange call site will cause quiescence to hang until the next activity notification. Reviewers should verify all session-input-producing paths in AgentSession notify checkpoint waiters in their finally blocks.

^{Macroscope summarized 5bcedd4.}

Note: trivial textual overlap with #1844 (both add cleanup lines to the same finally block in _runAutoCompaction; both lines should survive). Whichever PR merges second will be refreshed against main.

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 9593430. Configure here.}