DO duration alerter: one daily thread, no false "FAILED", sane thresholds

Status summary#

Spec'd 2026-09-04 from the first night of hourly alerts in #error-pulse (screenshot: prd breached every hour at ~540 DO-hours, dev/preview reported "probe FAILED to run" twice). Implemented on this branch; a forced 🧪 TEST RUN from CI produced the daily thread in #error-pulse (headline + 2 threaded replies + in-place headline edit), read back through the Slack API. Merged in #2590; the 37 hourly alert messages from the prior 48h were deleted from #error-pulse by hand after merge.

Why#

The hourly probe (#2576) posts one top-level Slack message per breached account per hour, so a chronic breach (prd's project-worker namespace, ~540 DO-hours/h ≈ $3/h since 2026-09-03 11:00) is 24 channel messages a day saying the same thing. And after #2585 (slots erased after every run), dev/preview legitimately has zero durableObjectsPeriodicGroups rows overnight; the probe's "an empty series is never quiet" guard now fires as "FAILED to run" every quiet hour.

What changes#

• One headline message per UTC day in #error-pulse (📒 Durable Objects — YYYY-MM-DD), found via conversations.history or created on first run; every alert (breach, pinned invocation, probe failure) is a reply in that thread; the headline is rewritten on every run, breached or not, with today's picture per account: latest hour, hours over the ceiling today, today's total DO-hours and ≈$, probe failures. (renderDailyThread + findOrCreateHeadline in scripts/ci/do-duration-alert.ts; a TEST RUN keeps its own thread)
• The probe reports every hour of the lookback in its summary (not only breached ones), and the alerter looks back 26h so "today" is complete.
• Empty series: prove the credentials and account with a call that does not depend on activity (the DO namespace list), then treat no rows as 0 DO-hours. Throw only when that proof fails.
• Thresholds: dev/preview 1000 → 500 ($2.80/h; healthy is 0–100 now, a relit slot is 2–4k); prd 400 → 600 ($3.40/h; the pre-incident baseline is ~100, with project-worker ~540 — under it, so the chronic case is visible in the headline without an hourly reply).
• Unit test for the headline/reply text from fixed summaries (scripts/ci/do-duration-alert.test.ts)

Out of scope#

• Fixing project-worker/IterateContextDurableObject itself (not in this repo; routed to its owner).