chore(gate): normalize the last future-dated stamp in task-update.ts (tightens the allowance 1 → 0) (#3168)

**Main is red on the FNXC gate again** — third occurrence of this class
today, third different file.

```
packages/core/src/task-store/task-update.ts: 2 future-dated FNXC stamp(s), baseline allows 1
```

Stamps dated **2026-08-01** while UTC is **2026-07-31-14:24**. Every
open PR inherits the failure; #3164 merged carrying it.

## The fix

Date only, to today. Clock times preserved exactly — they were real
times on the wrong day — and no comment text touched, so the record
reads identically, just in order:

```
-FNXC:StateMachine 2026-08-01-10:20 (PR #2793's finding — the INNER half, merged with #2821):
+FNXC:StateMachine 2026-07-31-10:20 (PR #2793's finding — the INNER half, merged with #2821):
```

Baseline **tightened** as a side effect (`1 → 0`): one future stamp was
grandfathered, normalizing the file cleared it too, and the gate refuses
a stale allowance on the way down. Re-recorded in the same commit.

## The recurrence is the point, not this fix

Three separate files have tripped this in one day — `scheduler.ts`, the
scheduler PG test, and now `task-update.ts` — plus the midnight-rollover
variant this morning that reddened everyone's baseline.

**Stamps are written from a local clock and validated against UTC.** A
worker behind UTC writes what is genuinely "today" for them and produces
a future stamp the moment UTC has already rolled. Nothing in the local
loop catches it: `pnpm lint` passes locally because the local date
agrees.

The durable fix is to generate the stamp from `date -u` rather than a
wall clock — one line in whatever produces these, and the class
disappears. I have patched the symptom three times today; someone should
take the cause. I have not done it myself because the stamps are
authored by hand across every worker's flow, so the change belongs
wherever that convention is documented, not in a file I happen to be
touching.

## Verification

`check-fnxc-future-dates` green (TZ=UTC CI=true) · `pnpm test:gate` 13 +
161 + 487 + 71 · lint · core typecheck clean · diff is date
substitutions only.

scripts/lib/fnxc-future-dates-baseline.json

@@ -32,7 +32,6 @@
   "packages/core/src/task-store/merge-queue-ops-2.ts": 4,
   "packages/core/src/task-store/task-artifacts-ops.ts": 6,
   "packages/core/src/task-store/task-store-helpers.ts": 2,
-  "packages/core/src/task-store/task-update.ts": 1,
   "packages/core/src/task-store/update-task-deps.ts": 5,
   "packages/core/src/task-timing.ts": 1,
   "packages/core/src/workflow-ir-resolver.ts": 2,