Fix heartbeat workflow trigger

[Pigbibi]

Summary

• remove the empty schedule event from execution-report-heartbeat.yml
• keep workflow_dispatch available for manual heartbeat checks
• document that unattended alerting is owned by trade/error notifications

Root cause

The previous workflow kept a schedule key with only a comment under it. GitHub accepted the file locally as YAML, but Actions treats the empty scheduled event as an invalid workflow trigger and created failed push checks with no jobs/logs.

Verification

• git diff --check
• uv run --with pyyaml python syntax parse for execution-report-heartbeat.yml