#12837: fix(heartbeat): suppress HEARTBEAT_OK token delivery

## Summary Fixes a heartbeat delivery edge case where model heartbeat ack tokens (`HEARTBEAT_OK`) could be relayed to user channels. ## Changes - In `runHeartbeatOnce`, when a heartbeat reply normalizes to an `ok-token` skip (`HEARTBEAT_OK` with no deliverable content), the runner now discards it. - `HEARTBEAT_OK` is no longer relayed to user channels in that path (even when `visibility.showOk` is enabled). - Updated test coverage in: - `src/infra/heartbeat-runner.respects-ackmaxchars-heartbeat-acks.test.ts` ## Why this is not a duplicate - Checked issue timeline and cross-references before starting. - No open PR was found that fixes #12767 directly. - #12785 references #12767 as a starter-issues list, not an implementation PR. ## Testing - Ran locally: - `pnpm test src/infra/heartbeat-runner.respects-ackmaxchars-heartbeat-acks.test.ts` - Result: **1 file passed, 9 tests passed** ## Contributing checklist - [x] Focused PR that addresses a single issue - [x] AI-assisted: yes - [x] Model used: `openai-codex/gpt-5.3-codex` - [x] Testing disclosed (targeted test above) Closes #12767