fix(slack): real root cause of notifier spam — preserve all NotifierState fields

Summary

This is the real root cause of the daily-brief spam (and the original trends spam). The Colima/virtiofs cache theory was a red herring — the previous anchor workarounds patched symptoms.

Four notify_* functions silently dropped state fields:

• notify_pending_approvals dropped last_trends_digest_at, last_daily_brief_date, etc.
• notify_latest_digest dropped same set
• notify_metric_anomalies dropped same set
• notify_readiness_change dropped same set

Since run_notifier chains these, by the time notify_daily_brief ran its dedup check, last_daily_brief_date was already reset to "" → the brief re-posted every tick. Same mechanism killed trends dedup.

Fix

Replace every NotifierState(...) reconstruction with dataclasses.replace(state, ...) so unrelated fields carry through. Adding a new field is now safe.

Test plan

• 4 regression tests pin the invariant (38/38 pass)
• APEX Review (semantic_reviewer @ 0.98 confidence) flagged the exact pattern across all four functions
• Monitor notifier for 2-3 ticks post-deploy to confirm no more brief posts

Credit: /audit skill caught this via APEX.