refactor(coding-agent): represent heartbeat cache validity by presence

[snimu]

Status: fully reverted — this PR is now a no-op. A cursor finding revealed the audit premise was wrong: heartbeat_manage intentionally consumed the retained stale snapshot as the routing index for unloaded sessions, and a successful manage synchronously emits heartbeats_changed before its response returns — so collapsing the flag into presence cleared the routing index on every manage. Base behavior restored in 3fb6ee2. The PR remains open only as a link in the stack chain; merge (empty) or close and retarget #1744 to snimu/derive-connection-models.

What was wrong

"Is the cached heartbeat snapshot usable?" was answered twice in the daemon supervisor: by the snapshot's presence AND by a heartbeatSnapshotStale flag, kept aligned by paired writes (audit: duplicate sources of truth, dup-truth.md finding 6).

The fix

The flag is deleted. heartbeats_changed clears the snapshot; a successful list sets it; validity is presence. All list branches behave identically under the mapping stale=true ⇔ absent. One deliberate narrowing: heartbeat_manage no longer routes via a stale-but-present snapshot — benign, because heartbeats_changed only arrives from a live worker whose broadcast immediately triggers client list refreshes that repopulate the cache, with findWorkerForClient as the fallback; disconnected-worker recovery is unaffected (a dead socket can't emit the event). +5/−7.

How it's verified

Reviewer proved the state bijection branch-by-branch, identified and vetted the one behavioral delta (correcting the audit's "stale snapshots were never consumed" claim), and confirmed no ordering race (frames on one socket are ordered). Heartbeats suite 6/6; full CI-style suite: no branch-only failures vs stack base. Two-model implement/review loop, approved first pass.

Stacked on #1741 (test the whole stack at the leaf; merge base-first).

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

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Note

Low Risk
No functional code changes in the diff; risk is limited to process/stack bookkeeping if an empty merge is mistaken for a behavior change.

Overview
This PR is effectively empty — the diff has no code changes, and the description states the intended refactor was fully reverted in 3fb6ee2a2.

The original goal was to drop heartbeatSnapshotStale in daemon-supervisor and treat cache validity as “snapshot present vs absent,” aligned with clearing on heartbeats_changed and repopulating on a successful list. That was rolled back because heartbeat_manage still relies on a stale-but-present snapshot as the routing index for unloaded sessions; treating stale as absent would clear that index on every manage before clients could refresh.

Current behavior is unchanged: heartbeatSnapshot and heartbeatSnapshotStale remain the dual source of truth in the supervisor. The PR is kept open only for stack linkage; merge as empty or close and retarget downstream work.

^{Reviewed by Cursor Bugbot for commit e4e12e6. Bugbot is set up for automated code reviews on this repo. Configure here.}

Linear ticket: ENG-5661
(ticket linked above)

Note

Refactor coding-agent heartbeat cache validity to use presence

The PR contains only blank line modifications. There are no functional changes to the heartbeat cache logic or any other code.

^{Macroscope summarized e4e12e6.}

[cursor]

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

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[snimu]

Closing: fully reverted after the routing-cache finding invalidated the audit premise (see resolved thread). The branch stays in place as history for the stack; #1744 has been retargeted to snimu/derive-connection-models.

[snimu]

Reopening: GitHub refuses base changes on PRs that are part of a stack, so #1744 cannot be retargeted away from this branch. This PR stays as a no-op link in the chain — merging it (empty diff) is the supported path, and its merge+delete will auto-retarget #1744.

[snimu]

Closing permanently: this PR is an intentional no-op (C7 was fully reverted after a review finding invalidated the audit premise; the branch carries net-zero changes). The stack is being converted to plain PRs against main, so this placeholder is no longer needed. Ticket ENG-5661 will be cancelled.