#15193: fix(heartbeat): keep scheduler chain alive on requests-in-flight skip

## Summary Fix heartbeat runner control flow so `requests-in-flight` means **defer**, not early exit. ## Problem `requests-in-flight` is documented as "skip now, retry later", but the current loop exits early in this branch, making scheduling continuity branch-dependent. ## Root cause In `src/infra/heartbeat-runner.ts`, the `requests-in-flight` path returns from inside the per-agent loop. ## Fix In that branch, switch to loop continuation (`continue`) so execution reaches the shared end-of-run scheduling path. This complements: - #14901 (scheduler continuity when `runOnce()` throws) - #14396 (busy-lane skip conditions) ## Validation - ✅ `src/infra/heartbeat-wake.test.ts` - ✅ `src/infra/heartbeat-runner.ghost-reminder.test.ts` ## Risk Minimal diff, no config/API changes, preserves intended "skip + retry later" semantics. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR changes the heartbeat runner’s per-agent control flow so the `requests-in-flight` skip path uses `continue` instead of returning early, aiming to keep the shared end-of-run scheduling path (`scheduleNext()`) consistent. However, `startHeartbeatRunner` is also the wake handler used by `heartbeat-wake.ts`, and that wake layer depends on the runner returning `{ status: "skipped", reason: "requests-in-flight" }` to trigger a short retry backoff. With the new `continue`, the runner can lose that skip reason and fall through to `{ status: "skipped", reason: isInterval ? "not-due" : "disabled" }`, which prevents the wake layer from retrying promptly under main-lane load. <h3>Confidence Score: 2/5</h3> - This PR has a functional regression in the wake retry control flow and is not safe to merge as-is. - Although the change keeps `scheduleNext()` reachable, it causes `startHeartbeatRunner` to stop returning the `requests-in-flight` skip reason that `heartbeat-wake.ts` uses to schedule a fast retry. Under load, this can delay or suppress retries and misreport manual/exec/hook wakes as "disabled". - src/infra/heartbeat-runner.ts _{Last reviewed commit: a23f144} <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->