Clarify silent Slack heartbeat response

[albert20260301]

Summary

• specify that any heartbeat whose active output surface is Slack must return exactly NO_REPLY when there is no human-facing Slack action
• explicitly include regular scheduled heartbeats whose result would otherwise be posted into a Slack channel or thread
• keep HEARTBEAT_OK limited to non-Slack/internal heartbeat invokers
• reconcile the communication tone rule with the Slack silence behavior

Rationale

PR #59 said routine heartbeat runs should be silent in Slack, but the 14:00 UTC regular heartbeat still posted a no-op summary. The missing operational detail is that when a heartbeat would emit to Slack and has no Slack-visible action, the final response must be NO_REPLY; this makes that explicit where heartbeat and Slack communication rules are read.

Validation

• bash scripts/validate-workspace.sh
• make lint

[albert20260301]

@redox this is the follow-up from the Slack thread: it makes Slack no-op heartbeats return exactly NO_REPLY so the channel gets no message. CI is green.